Hi, I can't really answer the datebase size against time question but happy to relay my experience. Our vCenter DB is around 30Gb and when we upgraded the vCenter portion (if I remember correctly) it was reasonably quick. I think it was about 15 mins. The longest part was the SSO install section at the start. I think the whole upgrade was around an hour. We did the upgrade at the weekend so time wasn't an issue for us
We were upgrading a MS 2008 SQL database hosted on a separate dedicated DB server.
The only real recommendation I would have is make sure you fully understand SSO and how you want to implement it into your environment.
Hope that helps.