If you want it for free, then teach Java. This code you will not find.
What need:
1. Create new items and itemhandler for activation premium time.
2. Create global task for control premium state of all characters. If you want global premium for all characters and servers when need cretate global task on the Login server side. Otherwise, on the Game server.
3. Add modifiers and checks for rates EXP, SP and DROP for premium users.
And this you want to get at free? Learn Java or buy.