You can share the Premium account for as long as u want but if u cross the 3000 MB limit per day then it will show the message that the Account is disabled/unavailable for the day.
Also, Rapidshare has a sort of I.P Address tracking system, for example, if a user from Computer A uses a Premium account and another user shares his Premium account details username and password from a new computer B, then (in most cases) u will get a file/downloading error so be careful for these pitfalls....