RE: battery duration
The standard receiver pack is 600 millamp HOURs. That means it should nominally deliver current at a 600 milliamp rate for 1 hour. A normal rate of discharge for 4 servos and reciever is probably on the order of 250-300 milliamps, assuming you're not banging the sticks around constantly. At that rate, you should have roughly 2 hours total flight time (300 milliamps x 2 hours=600 milliamp-hours)
If you fly for 10 minutes each flight (a typical flight time), you should be able to get 10-12 flights on a fully charged battery.
Now scale up to your 1500 mah battery. The math is left as an exercise for the student.
As for a cutoff voltage, your best bet is to use your radio on the ground and discharge the battery with the plane. Measure the voltage frequently and determine the voltage at which your servos become sluggish. Thats the voltage to avoid. If you've been measuring frequently, you should now have an idea of what the voltage was 10-15 minutes before you hit the minimum voltage. Add .1 volts and make that your no fly voltage.
As for charging, the typical wal-wart charger is not a constant current charger, but is normally a constant voltage current limited charger. That means that it delivers a maximum of 50 milliamps. As the battery approaches fully charged, the current is going to drop off. So your 1500 mah battery would require a MINIMUM of 30 hours (1500 mah/50 ma)to charge, and more likely will require over 40 hours to fully charge.
Many fast chargers allow you pick a charge rate, and most battery manufacturers recommend a C/10 rate, so if you can select a 150 ma charging rate, it should take about 10 hours to charge. You can probably go as high as a C/5 rate (300 ma) without worry.
Brad