The interest rate you pay after your savings have been offset is fixed for a set period with a fixed-rate mortgage, meaning your mortgage repayments stay exactly the same until that deal comes to an end.
Fixed-rate deals are beneficial if you need to stick to a tight budget as you know how much you need to pay each month during the deal. However, if interest rates fall, you won't benefit from a decrease in payments.
Fixed deals are typically offered for two to five years, but some lenders offer longer fixed-rate term options.