Your peak sounds good for the charge time. Most of the guys I fly with use this rule of thumb. It's not perfect and it does allow for a margin of error. others will have different opinions on this so that this into consideration as well and then make your own decision. Fly when your tester shows 4.9 volts but stop flying at 4.8 volts. As another poster stated, a healthy NiCd cell will hang a good while around 4.9 to 5.0 volts. Once it does begin to drop below 4.8 volts it can go very quickly depending on many things. You could still probably fly at 4.7 volts, but man, you are really getting very close to the danger point and it's just not worth the risk. When they go out they go very quickly.

Also, cycle your batteries about every three months and throw them away and replace at three year intervals or less but don't go beyond 3 years. Will they potentially last longer, yes! But, here again, you really need that safety margin and a battery pack is much less than an airplane cost wise. If the batteries are still good after three years break the pack apart and use the double a cells in a small flashlight to get your full value.

You may also want to get a good field quick charger that will usually peak charge your battery pack in about 30-45 minutes, but don't use this method all the time. I like the overnight method as a standard and then use a quick charger when I want to keep flying a littler longer. Just my 2 cents.