Can you use them at your destination?
As far as I know you can use the Virgin First Class lounge at the end of your journey if all or part of your journey was on a Virgin service and you hold a first class ticket.
I travelled locally from Coventry to Wolverhampton on a First Class Anytime Day Single - I needed to change trains at Birmingham New Street. I was able to use the lounge at Birmingham New Street.
I then bought another ticket at Wolverhampton - the same but to Birmingham New Street - and was able to enjoy the Wolverhampton lounge for about an hour before a Virgin train to Birmingham New Street. I think there were a couple of other trains but I missed them on the monitors.
The BHM lounge had a concierge who I showed my ticket to on entry - even though I didn't think he needed to - and the WVH one has an intercom with camera. Both lounges are very pleasant. I've haven't tried the WiFi yet though - didn't realise they had it until later.
I think I could have travelled back with another operator but thought it was only right to travel back on Virgin having used the lounge and time wasn't an issue.
My personal opinion about the lounges is that I treat them like I would expect someone to treat my own living room (sort of) - with respect and without abusing the privilege if you understand what I mean - not that I am suggesting for a moment anyone on here will do otherwise. I didn't expect to be able to use them on a local journey on a ticket which cost me less than £5 but I was made to feel as welcome as someone on an £100+ ticket which I didn't expect to be honest and was very impressed. I had the whole of the Wolverhampton lounge to myself for about an hour. The most enjoyable day out I've had some while.
As for how long you can stay in the lounge before or after travelling on a Virgin service - I think an hour is acceptable, more within specific reason.
Hope this helps.
Kindest regards,
Steve.