How long to run a cell can be a difficult question to answer.
It will depend on the amount of NaCl that you started off with, how low you want to decrease the Chloride concentration to (anode wear considerations), the current entering the cell and the current efficiency .
The current efficiency achieved in industrial plant (with pH control) is in the region of 95%. For the amateur it will be much less as pH control is not usually performed. With Platinum, Graphite, MMO and MnO2 45% to 55% for a typical set up without pH control will be achieved, assuming you are not running your cell to a low Chloride concentration, say 90g/l or less. Magnetite needs pH control for to achieve sensible CE.
With MMO good efficiencys are achieved even when the cell is let run to low (40g/l) Chloride levels. With Lead Dioxide low CE's without pH control are obtained.

The way the Chlorate maker should look at electricity that enters the cell is in terms of the total number of electrons that has entered the cell over a period of time (that's the total charge, that has entered the cell). Amps are the RATE of flow of charge. (like gallons per second is to water).There are a number of different units that are used to describe the number of electrons (charge). (This is similar to the fact that there are a number of different ways to describe, for example, length, the Meter, Mile, Foot, Yard, etc )

The Coulomb, (C) [pronounced cool-em]
The ampere hour,(Ah)
The ampere minute, (Am)
Ampere seconds, (As)
The Faraday, (Named after Michael Faraday) = One mole of electrons.
See What is a mole
All the above units are equivalent and are related thus:

The units we usually work in are ampere hours and (when we want to understand what is happening inside our cell) we may wish to talk in terms of moles of electrons.
When our cell is converting Chloride into Chlorate at 100% current efficiency it means that every six mole's of electrons that enters our cell is used to convert one mole of NaCl into one mole of NaClO3. It takes six electrons to 'stick' the three Oxygen atoms onto the Chloride.
When Charge is flowing at a rate of one Coulomb per second this is called one Amp. As stated above the Amp is the RATE of flow of Charge. The length of time needed for a current of one Amp to shift one mole of Electrons is 26.802 Hours.
Their is 58.4 grams NaCl (74.5 grams KCl) in one mole.

The following table is taken from Mike Browns page and shows the time for a Sodium Chlorate cell running at 100% current efficiency, to convert all of the Chloride in the cell to Chlorate.
If you run your cell for the amount of time in the (100% efficiency) run time table you will get somewhere between 45% and 55% of the NaCl turned into Chlorate for a cell that is not pH controlled. That is what we call 45 to 55% current efficiency. The other 45 to 55% of the electrons are 'lost' to other processes in the cell.

When your cell is working, some water is used up in the reactions and some will evaporate. This volume loss will have to be replaced or the cell contents will get small in volume and leave your Anode and Cathode not immersed as far as you would like. You can either add water or salt solution. If you want to separate Sodium Chlorate out of your cell as solid product then it is essential to keep topping up your cell with NaCl solution. You will have a high concentration of Sodium Chlorate in your electrolyte when you are finished which will give you some solid Chlorate ppt.
Know what the concentration of the top up solution is and keep a note of how much you add.
If you have a lid on the cell there will be little evaporation and therefor no room for adding salt solution.
If you run your cell with no lid, a Salt/Chlorate mist will decend on everything around the cell. The mist gets created by the bubbles of Hydrogen leaving the cell. It may be necessary to operate the cell outside or better to have some sort of cover that will allow evaporation but trap the dreaded mist.

It depends on what you are doing. You will also have to take into consideration what type of anode you are using, ie. if using Graphite the salt concentration should not be let fall low or excessive erosion will occur. If Sodium Perchlorate is the target, with no separation of intermediate Chlorate, then you will be letting it run for a long time and it is difficult to give a figure off hand, see the 'Sodium Perchlorate' section.
If you are making KClO3 by adding KCl, things are not very critical and you can let it run for the amount of time in the
Sodium Chlorate cell run time calculation. This formula will give you the practical run times for a cell that is working at various efficiencies. The formula insures that there will be about 100g/l of NaCl in the cell that is unconverted to Chlorate at the end of the run. This is OK, as it means that you will have gotten sensible current efficiencies throughout the run of the cell because you have not run your cell to very low Chloride concentrations where you would have gotten ridiculously low current efficiencies. It also means that you will not have eroded you Anodes excessively, especially Graphite.
You then add KCl, filter and start again. There will be 100g/l of Sodium Chloride left in the cell when you use the run times given by the formula. That’s why the cell volume is included in the formula.

For the person who wants to separate out solid Sodium Chlorate from the cell you should run your cell for the amount of time in the
Sodium Chlorate cell run time calculation. (same as above)
Include the NaCl that was added when topping up the cell in the run time calculation.

When removing product from the cell by crystallization, the appropriate mutual solubility diagram should be consulted. An explanation of how to interprete mutual solubility diagrams is in How to Design Fractional Crystallization Processes.
Also the book "Aqueous Solutions and the Phase Diagram" by FREDERICK FIELD PURDON and VICTOR WALLACE SLATER, is very useful reading. It is available in Google Books.

From Swede:
If you know the amount of dry solid product you have produced (grams) from a run you can calculate the CE using the following simple formula:

Process	Formula for CE in %
Na Chloride to Chlorate	151.08 * Weight/Ah
Na Chlorate to Perchlorate	43.78 * Weight/Ah
K Chloride to Chlorate	131.22 * Weight/Ah
K Chlorate to Perchlorate	38.69 * Weight/Ah
Na Chloride to Perchlorate	175.10 * Weight/Ah