If a buttermilk coffee cakes bakes for 40 min in 350F oven (35min convection oven), how long should I bake18 cupcakes of the coffee cake mix

Agreed. I'd start checking at 15 minutes by inserting a toothpick. I'd then increase in 5 min increments from there.

I think 15 or 20 minutes approximately half the baking time for the cake. I would start checking them about 5 minutes before baking time is up. When I bake cupcakes I find as a general rule thats about how long it takes.