Say I made a device that takes 50mA, and the batteries I use are 4 * AA (in serries) rechargeable NiMh, 2600mA.

Does this mean 2600/50=52 hours?

 

basically, that's it

 

Some good info in here:
http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf
It's going to make some difference if the battery's voltage affects the current that your circuit draws, but the most important thing is temperature.