battery drain

I am working on a simple alarm for the garage,

power total 650mA on my unit.

I am looking at using a alarm type battery that is rated for 12VDC and 650ma.
I would use a switching power supply to get the 5v so that there is no wasted power.

I have no experience with the actual power drain from such a battery. it would be on a trickle charge until power was lost (cut) and then once cut, the unit would go into battery mode.

any idea how long I could expect to have the unit run ?
are the gel-cell batteries rated based on near end of life ? or is that 5Ah rating only on a fresh battery ?

The battery referred to is 5Ah. This is a flexible figure as it depends on the state of charge when used. This in turn depends on the age of the battery, the previous usage, discharge current and voltage. Also how soon after discharge the battery is recharged.

A new freshly charged battery may last 10 hrs at 1/2 A but only 1/2 hr at 5 A. The capacity is normally rated at C10. This is the capacity at a 10 hr discharge.

The deeper discharge and greater number of them will degrade the battery quicker. Remember that running the battery flat will rapidly reduce the life of the battery.

In your case I would not want to discharge to less than 11.5 volts.

The battery charger can also effect the life of the battery. Over about 13.8V and you risk over charging it. At 12.5V it is not fully charged (while on float).