At my little college a professor has 3 years to get tenure. If he's denied after that time, he gets a 4th "lame duck" year. He is told that he is free to stick around as an adjunct instructor after that 4th year, but everybody knows he's not going to take it. It's just their way of saying, "Thanks, but this isn't working out. Take a hike."
My brother-in-law initially tried for an academic career in CS. After the third university denied him tenure (after a total of 9 years), he just went into industry. His kids were school age by then, and he just didn't want to continually uproot them. Industry has worked out very well for him, I might add.
Fortunately, he didn't have any graduate students that he was advising, because they would have had problems. (On the other hand, perhaps the lack of students was partially why he didn't get tenure!)
A friend of mine has gone from small NASA contract to small NASA contract (usually less than 6 months of support and that's a good year) to feed his family since being denied tenure. He worked for a year at a small liberal arts school and they just walked in one day towards the end of the year and told him he wasn't coming back in the fall.
He hasn't done much and is convinced he won't get another job.