I forget the trick that shows you how many hours. However, if this happens on a majority of tapes, then I would suspect you got ripped off and the unit needs to be repaired. This sounds like the classic R300 reel replacement problem. (there is a small gear that goes bad, Paul at Pro Digital can tell you more about the problem, and he and his crew can fix it, but most R300 owners I know have had to have their deck repaired for this problem -- including me).
Wayne