Welcome to the Cake Decorators Q&A

Hello binki

Vanilla sponge has a shelf life of about 5 days for maximum freshness but in reality it is usually fine for 7 days. If you brush each layer of your cake with simple syrup before filling and crumb coating the sponge will keep moist.
For simple syrup combine equal quantities of water and sugar and boil for a couple of minutes, cool and use on cake layers. You can also flavour the syrup if you want with a little alcohol or food extracts.
For a 8″ round x 3 layers use approx 200g of water and sugar. Adjust quantities for larger cakes.