Did the same last month, left our bags at the hotel spent the day in Porvoo as we had already had a couple of days in Helsinki. Got to the lounge about 6.30pm ( I did want to watch the Champions League final). The food is OK, the showers pretty good though the booking process has changed and basically need to line up but the bar is fabulous. There were plenty of other Aussies already there seeing most (all) of those midnight flights basically go to Asian ports.
We hadn't checked in but no issues checking in at the priority check in desk 5 hours before our flight and then getting access to the lounge. Didn't appear to be any time restrictions from what I could see.