I dump 2 liters of water in a 3 gallon fabric pot and that seems to wet everything nicely. Plants are happy and in need of water every 3 days.
For 5 gallons, try 4 liters of water. If you want some runoff then maybe 5 or 6 liters. Depends on how you apply the water too. If you just douche it in fast you'll get some that leaks through the fabric and out the sides. Pour it in more slowly and it will penetrate the soil better and more evenly.
If you are growing Photo Periods i would not plant a germinated seed into a 5 gallon pot right off the bat. You'll likely over water it and kill it.
If it's auto's you intend on growing, you won't need a 5 gallon pot. 3 gallon is a much better size for auto's because they don't grow a huge root mass. Your roots will likely never touch the sides of your 5 gal pot or the bottom.