Boring I know. Just a rough appreciation of how much is considered a sensible about. I've read about 10 times annual salary plus mortgage amount.
Also you have to decide how many years, how do you guess when your are likely to be pushing up daisies? Go with the biblical three score and ten or the UK average of 78.5 and hope for the best.
If you get a policy that covers the mortgage term, then do you take out another one if this runs out when you are in your fifties?