I understand how an open pitlane works, but without knowing the track limit, and the day limit, I've no way of figuring out whether its worth it.
If there are 100 cars there, 25 on track and 15 minute sessions, you're waiting 45 minutes a time :/
I've done 10 OPL days. The absolute maximum amount of time I've had to wait was 10 minutes at Bedford. We did OPL at Cadwell in March last year and I never had to wait more than once for traffic. There is no way in hell you'll have to wait for 45 mins.
The tracks are also particularly quiet at this time of year.