My understanding is that fitness losses are not linear and highly dependent on your physiology. However, "average" losses would probably be in the 30-40+% range, but I think you should be able to gain back that ground fairly quickly. I took 10 months off the bike to travel and within a matter of a couple months after resuming training was back to the level I was before. The important part is you gained that level of fitness before so it should be easier to get back to that same level of fitness vs gaining that fitness to begin with.