I take concerta 18mg. how likely it is to build a tolerance on this med? and do concerta "vacations" really work for restoring the effectiveness?

Concerta: Possibly limited effectiveness due to low dose. At higher doses, and for extended time, tolerance may occur, and some may benefit from drug holidays, although controversial.

Holiday not necesary: There is no need to consider a drug holiday for Concerta. Some people will seem to have a mild tolerance over an extended time, so speak with your prescriber about increasing your dose.