How rapidly does one lose fitness?

Having recently started paddling again, with the last time I paddled regularly (3+ times a week) being in February, I have started to wonder how much fitness one loses after such a long break.

My last paddles have been in the region of 6km each, about half an hour, and I was pretty tired at the end of it. Given that my goal is the Cape Point Challenge, a whopping 56km, I have a long way to go. I have to be fitter than 56km come December, since I believe one must always have a little extra in case things go wrong (e.g. a head wind, very choppy seas, swimming to shore). I'm looking forward to all the training, the time I spend on the ocean is always worth it, and it always makes whatever I may be worrying about look insignificant afterwards.

To get back to the topic, I think fitness dissipates pretty quickly, is dissipate the right word? My guess, if I had to put a figure to it, is 50% per month. So if you could paddle 20km, then do nothing for a month, you'll be able to do 10km, then after another month of no paddling, you'll be down to 5km which is pretty much rock bottom. Will consult scientific authorities when I have the time.