I went on WO sebatical () when I had my exams during summer.

Took a few (very intense) hours in TT to get back to where I was...If you're looking to get back into it quickly then I'd recommend chasing some of your ghosts on a track/tracks with complex/difficult lines...I think I did Sol 2 and maybe Vineta K (to get BRs/pitching on the go again)....not sure.
If you just want to get back into the game in general it might take a bit longer. I'd advise against going sub-phantom to get back into the swing of things (I know some people have tried this) because you'll get far too used to whichever class it is that you choose and your phantom game will inevitably suffer.

/two cents