minimum mAh for transmitter battery?

[Monkeysmate]

I'm after a little advice as to the minimum sensible mAh capacity for a transmitter battery.
I currently have 2900mAh NiCd in it, but i've got my eye on a very lightweight 700mAh LiFe battery for my lad.

Any thoughts?
Ta

[mattr]

I had a 1200 mAh LiFe in mine. That'd do an absolute shed load of running time. Double digit hours of actual use.

Only charged it when i thought about it. Don't think i ever got low voltage warning.