No standard answer. I've done work for free for at least 6 friends. Material and labor. They have all reciprocated, or spotted me first, or else I'm close enough where I don't care.
About 13 years ago I wired 2 new houses for free. One for friend who is a mason, another a pumber. I figured that when and if I built a house, they'd help me. About a year later they did. If I did not end up building too, it would have been OK, I felt close enough to them that if I never ended up building, I'd have still been glad to do it. I have a few other similar situations. I did do these jobs after hours (even though I owned my company) as I felt I couldn't afford to take away my everyday revenue.
Anytime I think someone's looking for a deal - they aren't getting it. Either I'm full price or too busy.
I do have some in-betweens where if I know someone well who needs something and can wait, then I might do it for a bit less than my going rate.