YMMV, but as a person who has been on ADD medication and started having to ratchet dosage up and up as my body built up tolerance, I started researching online for more information. I found that my situation was VERY VERY common for others taking ritalin/adderall/vyvanse-like drugs.

Very conspicuously, whenever I read "official" research on these medications, the studies were always short term like 6 weeks, maybe 12. One of the drugs I was prescribed had been out for years yet the only study available was for 6 weeks. Isn't that odd? Wouldn't you think that with the drug's having been on the market for years and with lots of online accounts of decrease in effectiveness after a few months that someone would have done a study? Doesn't the FDA require any follow-up studies be done after a drug is released to show a lack of side effects or continued efficacy?

It's really not surprising to me that now that medical practitioners are gaining years of experience prescribing these drugs that this long-term efficacy issue is going to become more of a subject of discussion.

There is certainly a potential to build a tolerance to stimulants, and that's one possible reason to switch drugs. As you said, it varies from person to person. For drugs with minimal withdrawal symptoms, like ritalin, it is also possible to mitigate tolerance effects by taking regular vacations from the medication.

Out of curiosity, have you had to increase your dosage significantly as an adult? It is common for people to need to increase their dosage during adolescence because their bodies change, and that's not necessarily indicative of a tolerance effect.