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

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.

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.