I don't know what percentage are registered on the whole. (11,881,376 possible LLLLL.com's)
But we can look at what has dropped recently and see what that shows to get an idea of how high demand might be.
Today: 1,439 dropped. Approximately 1,431 of those are available to re-register. (~99% available)
Over the past 7 days: 9,677 dropped. Approximately 8,695 available of those are available to re-register. (~90% available)
So there isn't a huge demand for random combos of letters for LLLLL.com's. Words, pronounceable combos, etc. may be worth something.
Now compare that to LLLL.com's (4L's) where there are 456,976 total possible combos.
Over the past 7 days: 0 have dropped.
Source: using ExpiredDomains.net