And how often? Once a month?

Bump.  I want to know as well....

I water about every three weeks.

How big is your pots? I lift the pots to know how much water they have when I store them. Sometimes they dry out more in the garage in warmer winters. Then again never water them as much as in the summer. Not even half that amount. Just enought to keep the roots alive. You should never have water in the saucer. I use one watering can for the two large figs in about 20 inch pots and my pomegranate in a 18 inch pot. I'll have to refill for my new fig in the small starter pot, about a 12 pot. I worry more about the pomegranate drying out.

Only when the soil is feeling dry...about once a month in the winter where I store them.  It stays about 40F all winter in that room off my basement.  I temperature will have a lot to do with the amount of water needed.

Should not take too much at all for a  fig without leaves. Maybe 12 oz or less. Start with a cup and see how dry they are in 3 weeks. Mine are still outside. Frost got the leaves this week. So I'll work on bringing them in after I put the foam boards back in the garage vent at the top of the gable end. I just closed the garage window last week. My naked figs got rained on today. So 4 weeks might be too soon for mine to be watered. Hate to think how heavy they are for me to move them in by myself.

Last year the temps. dipped to minus numbers a few days and I carried the figs to the basement for a week.  Wish I could leave them in the basement. Just too warm.

Depends on what kind of pot.  If you are using a cloth pot, root bag, that kind of thing, they will need quite a bit.  I lost 4 trees last winter because I was not aware of this.  They were just bone dry come springtime.  I bet a lot of the moisture wicked out the bottom into the concrete.  Maybe this year I will put down some plastic first. 

If you are using a plastic pot, I think once every couple weeks is fine.  And they don't need much at a time.  I like to use snow because it melts slowly so it makes less of a mess and I know all of it will get absorbed.  So each time I shovel the driveway, I give them a good inch or two of snow. 

As someone mentioned above, it depends on many storage conditions.
It is rightly said that not too much watering is needed during dormancy; just enough to avoid the soil being bone dried close to the surface.
My situation may be different for a number of reason so (caution) it may not apply to others. I am in zone 5a and store my plants in insulted garage kept just above freezing. I water well, really well, before storing. When I store my plants in the garage, they are tightly packed, pots on top of pots in a good size area so I do not have access to most of the pots for watering. So I do not water my pots from early November to the third week of April when brought out and I do not find them bone dried. My loss is less than 5% during winter storage and this loss include those plants which green-up later in June-July.