I read this article that put 99% uptime into perspective, and it was really enlightening.Alas my memory has failed me and I have no idea what happened to it or where I found it, but I did find this:
SLA level of 99.9 % uptime/availability gives following periods of potential downtime/unavailability:
Daily: 1m 26.4s
Weekly: 10m 4.8s
Monthly: 43m 49.7s
Yearly: 8h 45m 57.0s
That's around how much I'll tolerate of downtime if they promise me 99% uptime. Most hosts now have some kind of failsafe, so if a server goes down another one with the same data is already up and running, so you usually don't face downtime but that plan is horribly expensive.
As far as I've noticed, my web host hasn't had any downtime. I can deal with a little bit of downtime. Of course, 99% uptime is the standard. I expect my web host to fall somewhere close to that each month. There should never be more than one hour of downtime in a month.
I have high exceptions that my forum will stay up for majority of the month. Since, I wasn't paying for my last website and it was free hosting, I couldn't moan either way. The free hosting business had downtime once a month for a couple of hours and this could been because of a DDoS Attack, or maintenance problems with the internet host.
That quite an odd question as everyone expect as close as zero downtime as possible. But usually any website hosts worth their salts will compensate for any major downtime excluding scheduled maintenance, upgrades, and so on.