As long as you want them to. There are two answers to this. If you are talking how long will my initial rate last then you can have a rate from 1 year or a rate which can last for the lifetime (or term) of the mortgage.
If you are asking about the “term” of the mortgage then they typically range from 5 years up until 40 years. For the older client, there are options for the mortgage to run for your entire life span (these are called equity release or lifetime mortgages) More lenders are lending to older people with no end age so as long as you keep up the repayments on the mortgage it is only payable when you pass away or until you sell the property.