Pretty good I'd say, as the others said it's pretty contagious.

Like Tinkerbell, my DS2 had them at 4months too, I was BFing & he still got a pretty good dose.

I'm not sure how long it would take to show up, but DS2 got them about 2 weeks after DD1 came down with them & about 1 week after DS1.
So maybe a few days, maybe a week?...