[DonC]

My battery manager will supply up to a 2A trickle charge if needed to maintain the battery state.
I've got an Optima in the little red car and it has a 44Amp/Hour rating. That means that it will supply 44 amps for an hour before becoming discharged.
The trick is to gain 2 pieces of information.
1. What is the amp/hr rating of your battery. That doesn't mean CCA or any of the other values that manufacturers and sales outlets throw around but the amp/hr rating. You may have to dig a little to find it.
2. What is the draw on the battery of those items that are "always on". With a chaged battery in place simply hook an ampmeter (radio shack cheapy works fine) in line with the positive cable of the battery between the + post and the cable end. You will probably get a value in miliamps - 0.0xx or 0.00x (thousandths of an amp)
Take the mA rating and divide it into the amp/hr rating of the battery and you'll know how long the always on static drain on the battery will take to deplete its charge.
In my case I have a 0.03A always on static drain on my battery and, at 44 amp/hr it will take 1466 hrs (61 days) to drain the battery.
DonC