I believe (and I'll admit it if I'm found to be wrong) it behaves like chicken pox - incubation is several days to 2 weeks and then the pox show up. Once the pox show up is when one is contagious. That's why my pediatrician hustled me out of the waiting room and out the back door of the office when I brought my son in with this strange rash last year . . . .sure 'nuff - It was chicken pox (a year AFTER his chicken pox vaccination).
Smallpox and cowpox are trey Poxes, and are in the Variola family of viruses.
The characteristics of Chicken Pox aren't necessarily going to be a good indicator of the characteristics of Smallpox.