Unfortunately there is no simple answer to this as any number is meaningless due to the fact that we all have different time restrictions in our lives and different time management styles to deal with them.
Well one person may manage 6 properties relatively easy another may struggle with 3. It all depends on your commitments to family, friends, work, leisure time, etc...
Personally I jump at the chance to use any tool, process, or system that makes my life easier and gives me more free time so I would say that its beneficial even with only 1 property. (especially if you do your own book keeping and taxes)