Overdose of methylphenidate is supposed to be 700-1300mg at once. There has been a report in France where someone was taken to the hospital after taking 600mg at once and had the predicted effects but didn't die.

Also some people IV 1 gram of methylphenidate at once and then 1 gram of cocaine at once and say they are indistinguishable from one another.

Apparently someone took 5g in one day but that's just past down information.

I do hear about people taking 1 gram of Ethylphendate a day though.