If they aren't 'bone' dry, it usually works out to about 1/4 of the pot size. Like, 1qt for a gallon pot, 1/2gallon for a 2 gallon and so on.
This is just something I've noticed over time, I really don't measure, but my outside *watering can* is an old Osmocote jug that hold just over 1/2 gallon. I have 1, 2, 4, and 5 gallon pots and 8-5 gallon buckets of water for spot watering. For the whole deal, I use a hose.
Like BWD said, water till you have good run off.
If I let them get too dry, it might take more since I'll come back and hit them again after the medium gets moist. Like a dry mop vs damp mop. The damp mop absorbs water more readily than the dry one.
3 gallons plus 1 'just in case' ought to do you for 5-2gal pots.
Edit: A 5 gallon Homer bucket+airstone would be perfect.
Wet