Roknhs, you may not be charging with enough current. Check out these articles, they should clear this up for you.
http://www.masportaviator.com/ah.asp?CatID=2&ID=146
http://www.masportaviator.com/ah.asp?CatID=2&ID=149
Spitfire, flight time will depend on how many mA your plane draws. I have read that for a standard 4 servo trainer/sport plane you should figure about 110-125mA per 15 minute flight. You can figure the safe number of flights from there. Regarding when it's safe to keep flying, most people won't fly if their 4.8V rx battery shows 4.65V or less with a 250mA load placed on it.