1. No - However if you have lots of things running it will not remain idle for long - But personally I don't let it generate when idle or screen saver. I run it every two weeks as a part of my regular system maintenance.
2. This is entirely dependant on the amount of data not the size of HDD. VRDB is not a backup utility, it doesn't protect data files, mainly exe, dll, system files, etc. The first time you run it will take longer and I would suggest that you click the Generate VRDB Now! option and that should finish much quicker. Subsequent Generation should also be quicker, mine typically takes about 6 minutes