Calculating "useable amps"
According to the manufacturer's information, the 1650 maH capacity applies when the current drawn is 320 ma.  This would last for 1650/320 or 5.16 hours at which time the voltage will have fallen to 2.75 volts (again from the data sheet).  Since you plan to only draw 5 ma, the battery will last much longer even if your cutoff voltage is 3.2 volts instead of 2.75 volts.  A rough number is to divide 1650 by 5 which gives 330 hours but there's no guarantee of this.  Your best bet is either get more information from the manufacturer as to the battery behavior at low current drain, or make your own measurements. Apply your load and measure the battery voltage at intervals.  This may take quite awhile (days) but you may be able to extrapolate the data if it appears to be following a simple curve.