I know what you mean. I once requested flight following from Allegheny County Airport in Pittsburgh (AGC) to Westmorland County Airport (LBE). The two airports are 24 miles apart. I didn't realize until I started working as a controller how ridiculous the whole thing was, but as a new private pilot I felt a certain reassurance having someone guiding me.
I'd say 40 miles isn't unreasonable for a light single, especially on a VFR day when there's a lot of traffic out there. Unless I'm covered up with traffic, I never mind providing service to VFRs no matter how short the flight is. You'd be surprised how often I see VFR aircraft operating in close proximity to each other, whether they know it or not. So to answer your question, I'd say 30 miles or longer is perfectly reasonable to request flight following from a controller's standpoint.