Getting rusty happens gradually. As a very new pilot, I noticed the difference after even a couple of weeks. Recently, after a gap of six weeks, I went up with an instructor (club currency rules) and my flying was fine until we started practising auto-rotations (like PFLs, but harder) - and I'd have survived, had it been for real, but the helicopter might not.
And to put it the opposite way, when I was flying every day in the US, my flying was much better than after even a gap of a week.
But none of us can afford to fly every day.
So it comes down to knowing what works for you. Everybody's a bit different, but on the whole, the more experience you have, the longer gaps you can have before your flying really suffers. How you do that depends on you. Last year I kept my f/w flying up by circuit bashing for half an hour or so about once a month, then going off on a long trip with a friend in the summer. I just about managed the 12 hours. I wasn't brilliant, but I was safe. But I was doing lots of helicopter flying in between, and some things (like nav and radio) are the same, so that probably isn't typical. But it's one way of doing it.
I'd say think carefully before buying a share in an aircraft. Unless you plan to do quite a lot of flying it can end up costing more. Work it out first.