http://ratemyserver.net/index.php?mob_name=Atroce&page=mob_db&f=1&mob_search=SearchAtroce is one of the few overworld MVPs to ignore the typical 1-hour rule. @_@ Jeez @ those spawn times.
Even then, many MVPs have a dynamic respawn rate. This usually only matters on very populated servers; MVPs left alone for awhile spawn faster than if they were constantly farmed or fought for right on respawn.
This wasn't always the case; early on all MVPs had a fixed spawn time. Eventually though they got the dynamic respawn rates. It's about the same thing though, like for example 60minute MVPs became 60-90minute ones.
Now, this is all moot if the server specifically adjusted it to be a plain for-example 1hour respawn.