It's a meta answer. They are when they are.So when do you stop being an "Aspiring" drummer or guitarist and become a "Real" musician? When you play a gig, when you record, or only when you are a full time pro?
I can't think of a single guitarist I know that would have a problem with this. Those with perfect pitch would simply tune up the string that they know (for me, I can do a perfect A and E) and tune relative to that. Those without perfect pitch would ask for a key, or strike a fork.Also, give a lot of experienced guitarists a guitar that is wildly out of tune and ask them to tune it to "Concert pitch" without a tuner and most would struggle. Its easy to get a guitar in tune with its self, so to speak.
I'm not sure what your experience with guitarists has been, but tuning is a 101 skill. Even artistic guitarists (BB King) can tune by ear.