That is what I am using on my gas engines a 4.8v battery pack, 2000 mah.
I have run engines before for over a hour using just a 2000 mah pack too.

Now I do have some older ignition modules that draw more power though, so a larger capacity battery pack may be more useful for them.

What I doi is use a expanded scale voltmeter to check the battery pack after every flight at first. Then after I have determined it is holding up well and how long it is working for, I stop doing the battery checks so often.