Of course nothing will have 100.0 (repeating)% uptime. But 99.7% uptime means it can be down for over 2 hours every month. Anything less than 99.9% uptime (which means 3x less allowed downtime--a big difference) is probably unacceptable, and if downtime costs you serious money, you're going to want more decimal places.

Part of my job is network administration of a small (~50 server) colo/hosting service. It's unacceptable for us to be down for even 30 minutes (from our perspective and our clients). We maybe top out at 5 hours of downtime a year (during a bad year) and most of that (unfortunately) is upstream from us.