Guys, I got the reply from Poloniex support. Here we go: "ARDOR will indeed be credited accordingly with the number held on average during all snapshots. As I understand that would not require them to be in your account at all times or on the actual distribution date but I can not confirm that this is 100% the case. I do know balances will be credited and withdrawals of ARDOR will be allowed, but we are unsure of a permanent listing at this time." So basically I would hold my NXT's up to distribution date
It will be possible to start trading the ARDOR assets inside NXT asset exchange soon, but poloniex might decide not to give its customers their ARDOR assets, and wait until the ARDOR coin gets launched in six months before giving them to its customers.
I'm certain poloniex will credit customer's accounts with ARDOR, but I'm not sure if it will credit them soon or in six months time.