It's got to read every single page in that 700GB database, and do a bunch of checks to make sure all the pages are correct, all the DB structures are correct. An hour on 700GB isn't bad. For the 7 hour extremes, see if there's any IO contention at the time.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability