Hey gman, firstly the chemical in ritalin is called methylphenidate HCl
to answer your question, it all depends on which route of administration you choose as well as how often you do it. If you do it daily or take large doses, you'll find it takes more to get as high or to get high altogether. This effect usually wears off by abstaining from use for 2-3 days up to a week with heavy use. For insulfflation/snorting without tolerance keep doses in the
40-80 mg range. For oral administration keep the doses below 120mg, 150mg @most.
In terms of how long it stays in your system, methylphenidate's half life is relatively short, Around 1-3 hours I believe. "Half life" is the rate at which a drug is metabolized, thereby rendering it inactive metabolites. This can be explained with a hypothetical situation. Say you take 100 mg of methylphenidate, if it's half life is ~2 hours then in 2 hours only 50 mg will be present in your system, in another two hours you have 25mg, in another two hours, only 12.5 mg, so on and so forth. So for every 2 hour half lives of the drug that pass by, the amount is halved, Thus the term "half life".
However it is worth noting that an actual MPH (Methylphenidate) overdose is relatively rare, the reason I offered the above dose range is because many find larger doses uncomfortable and dysphoric (opposite of euphoric/happy) causing irritability, anxiousness, and causing insomnia. In truth 250mg+ doses are not unheard of, however such doses are usually taken by people with enormous tolerance. If you think you are having an overdose, do not hesitate to call an ambulance or get a friend to check you into the ER. As long as doses are kept reasonable and you don't mix high doses with other stimulants like caffeine, speed, or cocaine
I hope this answers your questions!!
Peace- Orangutang