Senza, you can't go wrong with charging at 1C.....This means that if you have a 4 cell 720 mah nimh rx battery, you can charge this pack at 720 mah on the charger. In essence, the pack will be charged in an hour. With nicad cells, you can even go up to 2C.
So with your 150 mah charger, the 720 nimh pack will charge in between 4 and 5 hours depending upon how much is left in the pack.

Dave...