Hello!, welcome to the forum.
It depends on a few factors....
You don't have to buy water for a fish tank, tap water with conditioner is fine. However, if you do have to buy water, I'd say spring. But I would just use tap water instead. Conditioner will last a while too!
Now, it mainly depends on the betta and it's home. As long as the tank is large enough, and heated, conditioned, etc, it could take from a couple days to a week to possibly longer. For me, it's usually been a couple days.
^^And I just had to say this!!! For my careers class in school, we just got done watching the movie "Pay it Forward" lol