Command time is primarily dependent on the rate of expansion of the airline. Retirements are relatively few as yet (Virgin has a fairly young pilot population), and it's quite likely that, before long, captains will retain their command until 65. Few people leave for other reasons. Therefore the primary source of new left-hand-seats is new aeroplanes!
At the moment, VS is taking, on average, three or four new aircraft a year, net. Some years more, some less. That rate looks set to continue over the next few years. Whether the company will continue to expand at this rate (>10% pa) is moot, but common sense would suggest that it can't continue indefinitely. At the current crewing ratio, and taking all other factors into account, VS needs about 40-50 new captains a year. There are around 400 FOs on line, so the maths is fairly straightforward - at the moment!
Looking at BA, whose fleet strength is static at the moment (and has been for some time), longhaul commands would seem to be around 23+ years right now, give or take a bit. It's therefore reasonable to suggest that VS time to command will tend in that direction as the airline gets bigger, and the proportional effect of three or four new aircraft a year decreases. The airline is forecast to be double its 2003 strength by 2012 (or thereabouts!); by then I would expect command for new entrants to take around 13 years. But who really knows?
Scroggs