I think this confuses many of us!
There's a term called Visible Lean (VL) which you will see used in many recipes. For example they will say "Pork Shoulder (80/20 VL)"; this is as it says 80% Lean and 20% fat and is generally what to tell your butcher to get the correct amount of fat on your meat.
As Hobbitfeet says it varies animal to animal: breed to breed, and by age. You have to take your best guess - particularly hard with shoulder where much of the fat is on the outside. Belly is easier.
The fat content will actually be about 8% higher than the visible fat due to fat present 'inside' the meat.
Average benchmarks for Pork (off the top of my head - so someone correct me if I'm wrong!) are Shoulder 15 - 25% visible fat, Belly 40 - 60%, Loin 15% - or thereabouts - even 100% back fat isn't actually 100% fat as it contains water!
There are some pictures for reference here:
http://home.pacbell.net/lpoli/page0001.htm
There are also some details here:
http://forum.sausagemaking.org/viewtopi ... t+fat#2097
I hope this helps
Phil