Using Campden in water

If you are using 1/4 campden tablet per 5 gallons, you should be fine. If you are using 1 tablet per gallon, then you need to wait 24 hours. I typically use 1 tablet for ~10 gallons prior to mashing and do not wait at all.

http://www.howtobrew.com/section1/chapter4-1.html

Are you adding it to the must to sterilise it, or are you simply adding it to the water to remove chlorine before adding the water to the other ingredients?

If the former, yes, you should leave it 24 hours before adding the yeast. By that time the sulphur dioxide will have evaporated and should be of no detriment to the yeast. I usually add the Campden when the must is hot, then after 12 hours add pectic enzyme, then after 12 more hours add the yeast (which I usually start in a starter solution at the same time as I add the Campden). This allows a gap between Campden and yeast for the sulphur dioxide to evaporate, a gap between boiling and adding pectic enzyme for the must to cool, and a gap between pectic enzyme and yeast so the enzyme can extract some of the flavour and colour before the yeast gets going.

I have less experience with the latter, but I understand that you use a lot less Campden per water. Also when you pour it into different containers that will aggravate it and some of the gas will be lost. So for both these reasons I would estimate that you would need less time before it is safe to add the yeast than you would if using the Campden to sterilise. However, if you are using the Campden only to remove chlorine, are you doing something else to sterilise the rest of the must? You mentioned in a comment on the other answer that you are not boiling the must. Be sure to remove the natural/airborne bacteria and yeasts before you add your yeast, because you have after all created a perfect environment for bacteria and yeast to grow in :)

Used as you are using it, it will all be gone by the time you add yeast so there's no problem. Not only are there hours between when you add the campden and the yeast, you'll be boiling the water before adding yeast. I have used it to inhibit bacteria when I make cider, but 24 hours after adding I stir the juice and the campden off gasses and there's not enough left to have any impact on the yeast.