A mortgage offer is confirmation that the lender will provide you with a mortgage. This will be after fully assessing your circumstances and a full application is made alongside a valuation of the property. The amount of time it takes to get to this stage depends on the overall complexity of the application. In general, it will take between 18-40 days for the processing of the mortgage application and an offer to be produced.
Mortgage offers can last between 3-6 months, depending on the lender. Some will be valid starting from the date of application whilst others start from the date the mortgage offer was created. It is worth bearing this in mind before applying.