That's probably fair - but it could be quite a bit less.

And remember - batteries lose capacity with age...


Why not?
It's usually written on the battery!


You said you are regulating the 9V battery down to 5V; as I said to you before, if you're using a linear regulator, then you are throwing away nearly half of the battery's capacity!!
http://www.8052.com/forum/read.phtml?id=135679


See above - do not use a linear regulator!

The way to get the most out of a "battery" is to use a single cell and a switching regulator.

A series-connected battery, like a chain, is limited by its weakest link - the overall battery capacity will be limited by the capacity of the "weakest" cell.

The trouble with a 9V battery is that it takes a lot of cells to get the voltage up to 9V; six 1.5V cells, or seven/eight 1.2V cells. Therefore, each cell has to be small to fit into the space. Therefore, the cells can only have a small capacity.
You compound this problem if you waste nearly half the capacity in a linear regulator!


In series, the mAh is limited by the actual capacity of the weakest cell, but you get higher voltage;
In parallel, the mAh capacities add (in theory, at least) - but the voltage is not increased.