This happened to me last year.
You can keep playing as long as you keep the game on. You can pause, walk away, whatever, but keep the game on. I was able to extend my preview for several days after my season ticket expired this way.
However, the game eventually froze. When I had to restart it, I couldn't get back in without purchasing the new season ticket.
So that's the trick: Keep the game on 24/7. If you shut it down after it expires, you'll be locked out.