10-24 days, according to the BBC website, but most say 10-21.
dd1 was exposed (not deliberately ) dozens of times, and only just got it at 4.5 a few weeks ago.
I knew it was going around her pre-school, and was worried about dd2 getting it worse (GPs say siblings often do, due to exposure to high viral load) as she is littler and has laryngomalacia. I did think about exposing her deliberately, so it would be milder than if she caught it from her sister, and honestly didn't know what to do for the best. Pre-dd2, I wouldn't have considered deliberate infection.
One of my MN pals helped me decide not to, so I didnt'. I then spent days worrying about how badly she was bound to have it (her sister had it fairly badly), due to imossibility of quarantining 4.5 yr old from 18 mo old, thinking I'd done the wrong thing, and guess what: she hasn't caught it at all
I'm glad, tbh, I'd rather she was a bit older or am even thinking about immunization, in her case, but the whole thing made me less black/white about whether or not to deliberately expose.
CP is a dodgy little beast, IME, unpredictable. And sometimes uncatchable.