Frankly I do not recall a single service without downtime, this includes banks I use. Yes I'd be mightily upset if said downtimes had lasted for days. 15 min - I do not give a flying hoot as long as it is not too often.
If you've got planned maintenance that requires downtime then you are always scheduling it at the times when your traffic is at its lowest. How much you avoid hard downtime is a function of how much money you're willing to spend on the maintenance.
I would literally have no idea if gmail went down from 1-2 am any day of the week. Hell. I wouldn't notice if it was down everyday from 1-2am.