You know how to study efficiently so you'll nail it easy XD And seeing as you know a bit already it will come back easy and anything you don't know you'll pick up quickly :)

I'm in this annoying phase where when I'm eavesdropping overhearing things I can pick up words but usually not enough fast enough to understand the conversation (though I was pretty happy at understanding most of the Aunty at the Chinese restaurant we were at for lunch even if she was just yelling our order to the kitchen XD), but when I'm reading stuff, while I'm recognising some characters enough to translate things I'm not actually reading what the characters are x_x

As for how much you forget well given my previous experience with a conlang I was actively trying to learn for a while and my parents and aunty (all of whom can speak Malay, I think my dad knows some Cantonese and my mum apparently knows Mandarin, Cantonese and some Hokkien according to one of my Chinese friends as I don't know enough to tell the difference), you retain whatever is important (so colloquial speech for the most part, and things to do with food and probably bad words XD) and whatever wasn't (anything beyond basic manners) goes XD