I use inches for a guess. About a foot, about six inches. There's a six foot timber lying in the skip, isn't there?
MM for whenever something's got to actually fit. I was very surprised last year to find young professionally trained plasterers using inches. I was using mm 30 years ago.
We had a young fella on site who claimed to be a joiner, didn't understand mm and claimed to be working in inches.
We never did find out if that was teh case because the silly boy turned out to be an ardent IRA supporter and - on a site staffed by two ex squaddies - he was running for his life by tea break.