The answer is usually 25 years. But this is only because that was the traditional length.

You can get a mortgage for any term. 15 or 20 year mortgages are fairly common.

The reason why people would want a shorter mortgage term is that - despite seeming to be more expensive - ie the monthly payments are higher - at the end of the day you'd be paying a lot less interest over the length of the mortgage.

There may be a problem with getting a mortgage which will last beyond your retirement date.

Again this is down to the traditional approach by the lenders (ie they assume you will retire at 60 or 65 on a modest pension so won't be able to afford the mortgage payments.)

But the traditional game is changing radically so - if this is an issue - shop around and you should be OK.