Well Scroggs, that all depends on the period of time which elapses between graduating - and either not finding a job, being fired or retiering
If one graduates, and keeps reading articles from for example the institute of physics, and takes a look over the subjects done at university (because lets face it, you have a degree in it, it must interest you!) This would DEFINATLY keep it pretty fresh in the mind. surely people have a couple of hours each fortnight etc..
Comeon - if i was a graduate, and didn't do anything with my degree, for sure i wouldn't forget it all! I do 101% disagree with you on that note.
I do think that it would be easy to keep 'up' on the theory, but the practical side of engeneering would be a hard point, however to tutor privatly (e.g. science/maths), and earn a healthy wage ..... there is NO practical stuff involved, for a fact.