Based on my tree alone, I think the answer is forever. Lol.
But to be fair, my tree has been in-ground 2 or 3 years from being a 3-gal, and it is a good 8ft tall now, and I never give it any special attention or extra watering or fertilization. It's big and healthy, but no flowers yet. Really hoping this is the year!
On the other hand, I had sugar apple which fruited at only 9 months! So if you're looking for fast, go with sugar apple (but do try to let it be 1-2 years old at least before you let it hold fruit, if you want it to last and be healthy). Otherwise, guanabana will take I've heard an average of 5 years.