No there is no easy answer.
Best bet is to have a read up on the particular model your Son is after, chances are the Manufacturers web site will have info on Battery life for specific models.
Some cameras will soak up power faster than others, if there is no moving parts (cheaper models) ie: electronic zoom lens, then the batteries will last longer than a similar model which does have a zoom, if a camera has a constantly on view screen it will use more power than one that does not, flash usage will wear them down very quickly.
To extend Battery life to it`s maximum switch the auto playback function off, keep zooming to a minimum, take the batteries out when not in use.
If you decide to get rechargables get a high rating set, but check which your camera uses, AA`s are usuall NiMH or Ni-cad, look for 2000Amh+ .
Shooting indoors using playback and Flash leaving the camera on while messing around getting things right i have only taken 40-50 photos from a full charge, while the same fully charged set used outdoors in good light has taken 200 shots, uploaded to the computer, then taken another 200 shots, so no definitive guide.