WN
This really is a "it depends" type of question. It depends on the airline and its cost structures and route network and it depends on the nature of demand and yield.
Ryanair's BELF in 2000 was around 60%, while EasyJet's was around 70%, in the US I understand BELFs went into the 80-90% bracket after 9/11 when they had to cut fares to get people flying.
If you're interested in reading up on the subject, try Handling the Big Jets by Paul Clark.