I don’t want to overwater.
I guess it depends on soil. Mine is well draining enough I can poor one or three gallons and in 15 minutes you would not be able to tell as both should saturate soil. I never had issues with too much water outside. I need it to hold a lot. The DE instead of perlite I use is holding an extra day of water for me. I’m at my cottage 35 miles away. I just watered today. I’ll go back home Thursday and water again . No rain for 7 days here. They will get very dry but should hold on till I get home Thursday. In five to twenty five gallon mostly cloth pots.
Your soil should drain off excess water so you can’t over water. In you do that hourly well yes otherwise no. I would use at least a gallon.
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Depends on the weather and how rootbound it is. A good rule of thumb is to water until it drains freely from the bottom, then wait until the top inch or two of soil is dry before watering again. In hot weather a 15 gal can need watering daily, in cooler spells every few days is fine.
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Before fruit starts to ripen, it’s hard to over-water a fig. You do want to have soil that drains OK while retaining moisture. A mix that is (a) 60-70% fine organic material such as compost, peat moss, and/or coco coir plus (b) 30-40% coarse but water-retentive material such as calcined clay and diatomaceous earth works very well.
In cool sunny weather without rain, I’d water every other day. In hot sunny weather, every day or, better yet, twice a day splitting the daily amount into two portions. Fig trees can easily survive with less water, but they won’t grow and fruit well. Once fruit starts to ripen, it’s best to cut back the amount of water somewhat. But even then, the solid should not be allowed to dry out.
Answering your question directly, I’d think that 3/4 gallon per day for a 15 g pot is plenty when fruit is ripening. But during midsummer heat before the fruit starts to ripen, I’d personally go with twice as much.
Opinions vary, of course. 
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In my environment, I add about 1⅓ gallons per week to my 15 gallon potted trees. Keep in mind that my soil mixture is 3 parts cured greenery waste + 1 part all purpose sand.
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Yeah I’m kinda old school. I water when needed. It really depends on so many variables. I do some by drip lines on timers and some by hand. Plants more sensitive to drought get the drip lines. Like tomatoes and peppers. I will adjust timers according to environment and local conditions. Always seem to be adjusting.
Watering a potted tree until the water freely drains from the bottom can be deceiving. If they are quite dry, lots of the time the water just follows the small space between the soil and edge of the pot and then drains out the holes in the bottom. The mass of dirt and roots can be bone dry as water is pouring out the bottom.
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I have had that happen before with plants Ive forgotten to water, for those I slowly water with a bit of water to rehydrate the soil a bit then come back later to fully water them
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