Number of laps depends on category (between 3 and 5) and tend to be around the 25 minute mark (but yes, depends on how fast you go!)
To be fair there are huge differences, at Wasing this year the fastest lap was 16:49 and plenty of folk doing >40 minutes. Middle of open was about 25 mins, that’s probably about the mean, but a relatively meaningless number.
They’re BC sanctioned, so the Elite race has to be between 1hr 45 and 2 hours for the winner, so unlike a Gorrick where some races will be long and some will be short they’ll chop and change laps on a Southern XC to ensure it’s on target. Expert is then meant to be 1:30-1:45 IIRC.
Bit more technical too, particularly in the last couple of years they’ve started putting new technical bits in.