How long does it take for my tolerance of ritalin (methylphenidate) to go down?

Few months: It depends on the dose you were on when you stopped and how long you were on it. Usually when people take a few months off (summer break) they may be able to start again at a lower dose (20 to 30% lower), but will quickly need to increase back to their previous dose (within a month). That's just my opinion. You might get a different answer from another doc.