Hi R. - What I did to transition my now 4 year old son is to start topping off each bottle of breastmilk with whole milk during the week. Gradually my supply went down while I was pumping and the bottles were filled more and more with whole milk until finally they were all whole milk. I continued to nurse him on the weekends until he gradually gave it up (I don't recall exactly when, but I think it was pretty fast that he lost interest in nursing during the day) and I also nursed him every morning and evening until he was 2.5 years old.
I wanted him to wean himself, but it seemed like that was never going to happen and we were not able to conceive another child until the first was weaned so on his 2.5 birthday we had a party and said no more "ba-ba" after today and he was fine with it.
SO, to answer your question, your supply will adjust and should continue as long as you want it to. I'd say it took about 2 months for my supply during the week to gradually reduce to almost nothing for pumping and I think at a year, he was only nursing 1 - 2 times during the day on the weekend (not including the morning and evening sessions).
Good luck and congrats for nursing for this long!