How long to bake a 12" chocolate sponge cake

[TheSkiingGardener]

I think the title says it all! I'm going to use the Hummingbird bakery's chocolate fairy cake mix and scale it up, but how long do you reckon a 12" cake would take? (Party is Sunday, please let it be less than 5 days....)

And does anyone know how I can make a template for a big number 4 to go on top. It's going to be a racetrack so needs to be a big fat number 4!

I actually cooked a 12" round today, slightly different to yours though in as much as I dont cook my cakes in one tin, I do it in two stages, dont know but have always had probs cooking a huge cake in one go, thats just me though ,my cake today was 12 eggs, 6 eggs each time and cooked them for hr and 5mins, gas 2,3.