I’d say years and of course it’s hugely dependent on your frame size and that of the fist. If you’re 6’4” and your fister is a 4’11” gymnast, you’re already off to a great start.
if you’re already playing with dildos in your butt, the chances are you’ll be quicker as you’ve already made a start.
as others have said, the key is repetition; a gentle but insistent stretch becomes less of a strain each time. … if you always remember to warm up, start gently using gently tapered, smooth toys and lots of slippery lube.
you can go deeper and so expand the stretch each time little by little without having to keep upsizing your dildo every few weeks.
try and use flexible toys especially as you get towards fist size as your fist isnt rigid nor perfectly round either.
when you think you’re ready to try a real fist, start with your big toy and then switch over to a very slippery hand and see how you go.
if you practice regularly and often, you should make good progress but this is still going to take months not weeks as you’re stretching & training a muscle. It needs time to stretch further, recover and stretch again.
have fun!