ROVI's = Retention Of Validation Instructors. These are valid (no jokes please!!!) controllers who work the majority of their time instructing student controllers in Hurn and return to their operational unit to maintain their validations. I think they require 16 hours per month.
Generally speaking a controller must exercise his validation within 30 days, go over that and they and their Local Competency Examiner (LCE) must have a little chat about whether they are up to it, new procedures etc and the LCE will make a decision on whether he/she is happy for the controller to work. If you go over 45 days you must sit with an examiner until he/she is happy for you to go solo again. Over 90 days - you've lost your validation and must retrain and sit a full validation board.
This is London Terminal Control, others will differ. There are also some other little rules and caveats that I wont bore you with.