There is no rule of thumb.
I have a read and studied this problem for quite a bit. And I have learned enough to know that I still have a lot to learn on the topic. But what I can say is that there can be a lot of variances (it is not an isotropic material) between balsa, and what one lot of balsa can do the other may not. That's a design problem that has a lot of uncertainty associated with it regarding unreinforced balsa. If you think you are going to go that fast, go with 1/8" plywood and the risk goes down. A lot (IMHO). At least paper the balsa, or better yet fiberglass it (that's what I do).
Greg