As long as your document(RTD/Permit) is good.RTD/Permit is not only used as Passport.This is also a visa to Reenter the US.That means as long as your RTD or Permit is good,you're allowed to reenter the US,no matter how much time did you spend away.Just make sure you come back to the US before your document(RTD or Permit) expires.If you spend more than 6 months away as a PR,this is gonna affect your citizenchip period of time.If you're not yet a PR and want to travel with your RTD,this doesn't apply to you.If you're a PR and want to travel with your National Passport,you're not allowed to stay away for more than 6 months.