It depends on the friend. I've just been waiting for about three weeks for a tile tapper friend to reenter the game. He's back and I'm glad about it. :)
Max level players with lots of premium content usually don't just drop out. I know, I wouldn't.
Somebody mentioned somewhere, that there is a way to send messages via origin. That would be great!
During my holiday a RLfriend tapped my houses and visited my friends. But that I quite a sacrifice, since I have 75 friends now and visiting them takes its time.
I would really like to be able to inform everybody if I'm away for a time.
Guess I could do it with some scrubs, but that would mean rearranging the whole landing zone and planting an "unde repair"©anettemarc as wel as an ooo...