If I mix Marmite and butter together, as Nigella does in How To Eat (p505), how long will it last in the fridge?
Marmite usually has quite a long shelf life so it is unlikely that the Marmite element will be a limit on the life of the mixture, though we would suggest that you do adhere to the "best before" date on the packaging. The butter is more likely to dictate the keeping qualities and the wrapper around the butter will give a "best before" date which will be a good guide to how long you can store a Marmite butter in the fridge. If you have just bought the butter then it is usually 4 to 6 weeks in the fridge, but about 1 week at room temperature (depending on how warm your kitchen is). Butter will turn rancid eventually and will not taste very nice.
If you are making a good quantity of Marmite butter to keep then you may find it useful to form the mixture into a log and wrap it in clingflim (plastic wrap) before refrigerating it. You can then slice off small pats of the butter as required. The Marmite butter does need to be soft to spread evenly and small pieces should soften fairly quickly at room temperature. The butter log can also be frozen (pre-sliced into pats, if preferred), tightly wrapped in a layer of clingfilm and a layer of foil, for up to 3 months.