Chargetime is not related to the size of the vehicle the battery goes into, but is based on the size of the battery.  Same size and rated battery will require the same chargetime regardless of what it goes into (1/8th, 1/18th).  Runtime however will be greatly altered within different size rc's, assuming your using the same battery for both.  Runtime is based on many factors: motor, servos, weight, esc, etc......Chargetime is ONLY based on the size of the battery itself, the rate at which it is charged, and how much has been drained from the battery prior to charging.