How many hours does a 1650 Mah battery last?

Depends on the draw of whatever you've hooked it up to.

If the draw is 1 mA, then it'll last for 1650 hours.

If the draw is 10 mA, then it'll last for 165 hours.

If the draw is 100 mA, then it'll last for 16 hours 30 minutes.