It depends. You can look back at a year and say "We had 100% uptime". You can also make an assurance that you have %90 uptime, %99, 99.9, or even %99.99 and beyond which all actually carry a meaning about load balancing, fall over, and redundancy.

How so?

> uptime percentages are a useless measure

Uptime is only useless if you don't care about high availability. Many startups don't, but Google sure does.