How many hours will it take to fill a 309 gallon hot tub

It depends on how you are filling it. If you are just putting water in by a hose, it will take some time, but not too long. If you are doing a bucket brigade, it will be even shorter. If you happen to be doing both, it will halve the time it takes to begin with.

The hours will vary. You have to take into consideration, "the amount of water", "the water pressure" and "what tools am i using to fill it up?"

So the hours will vary.