How long to wait after adding campden tablets?

I've been using campden tablets to dechlorinate my brewing water. I've heard that if you begin mashing too soon after treating the water it can cause problems with the wort. Additionally, I've heard that treating your top-off water and then adding it to the wort immediately after can stun the yeast. My question is: How long do I have to wait for the campden tablets to do their thing and no longer be a risk to my beer?

Its nearly instantaneous.

Thanks! You've laid some of my fears to rest.