The traditional length was always 25 years as others have said. We tied in with a 10-year fixed in '07, think that's still the longest they'll give you, as the rate was good. Then Northern Rock went under and for ages we were paying over the odds. The day came that we would could finally move. Decided to keep paying the same each month so we were overpaying. Probably the only sensible financial decision I've ever made.
We toyed with being mortgage free early and staying put but bought the new place last year. Another good decision. If things go to plan, it'll be paid off before the eldest hits 18.