They are designed by an engineer. It depends on the weight they are supporting, the type of beam used(steel, laminate, 2x10, 2x12), the distance being spanned. There is no such thing a rule of thumb for this.
I was eventually going to ask the same question. I have 2x6's beams and I think they were originally placed every 4-5ft under the 2 story section and hardly any under the rest. I can't tell for sure since some are rotted out.