Saffy - I've always worked on the very rough guide that:
50% is most people or first year or running
60% is decent - involves some training
70% is county standard so pretty good (will pick up the odd parkrun win but not amazing)
80-85% is national standard (for women you can make some cash at this level with race wins)
90% + is international standard (you can probably make a living from it at this stage as a woman, but unlikely as a man as there are more of you).
I have worked hard to go from 49% to 70% in 2 and a bit years...it gets tougher as you get higher (ever decreasing return on investment basically), but a runner at a decent training level should expect to improve for 6-10 years believe it or not.
It takes most people 8 years to reach their peak I've been told (from scratch)...I'm hoping to do it in 6 but I am relaxed about it taking longer if necessary 
Everyone can work off their own idea of where they would like to be and dont forget that you can start with 5ks and then aim to balance your wava over the longer distances (which is more difficult).
Am sure others will have different interpretations of course
Edited: 22/03/2011 at 13:41