Yes if you pay for a cloud system. Shared hosts can go down from time to time. Our servers have been up for about 60 days. The last downtime was scheduled and lasted less than 1 minute.
100% should be the norm for any cloud system. Even traditional server systems should be darn close to 100%. The only downtime we have had on any of our servers over the last several years, was 5 minutes here or there to install more RAM, throw a new hard drive in, etc. That's it. If you are constantly rebooting servers or experiencing downtime, there is something very, very wrong.
Like others have said cloud is the only way you can guarantee 100% uninterupted uptime. VPS and Dedicated servers are very close usually around 99%