Originally Posted by corporate-wage-slave

It's usually in the 12 to 24 hour range. On some occasions you can select direct into First, if you haven't printed your boarding pass, sometimes people will be moved there by ground agents and there won't be anything you can do about it. It typically won't be before then unless there are equipment changes or other factors that mean it will be opened sooner due to CW loads. If I suspected this was going to happen I'd check in at the earliest point but perhaps defer boarding pass downloading.

First only opens up if it's the only way to get all CW passengers seated. It takes up too many crew members for it to be done "just because we can". Furthermore there is always a good chance of an equipment change, LGW has very little spare aircraft capacity and LHR isn't overly blessed either.