They've got all kinds of other geeky stuff on the site if you start searching and can look at all the variables such as temperature, discharge rates, number of cycles etc., but I think the graph is the best explanation of what's expected.
Here's a good thread on performance if you want you NiMH.
It's been a while since I used lithiums in my GPS but I seem to remember that they would last around 36 hours in an Etrex H. I now use 2500mAh Eneloops and get 28 hours or so in an Etrex 30. Lithiums are much more expensive (per charge) than NiMH, but much lighter and don't leak if you leave them in the device for a long time. They also have a long shelf life so are good to carry as spares.