You can normally start any sanding and buffing process a few days after the paint has been sprayed.
The sooner you start sanding and buffing "generally speaking" the easier it will be to remove your sanding marks because the paint will still be more polishable. I wouldn't say it's soft but I would say it will not have reached full cure and hardness. What I have seen when paint is sanded and buffed too soon is the surface appearance changes for the worse and you'll see what looks like dull splotchy areas, sometimes you'll see sanding marks that you "thought" you removed re-appear.
The longer you wait, the more the paint will dry and do any shifting it's going to do and then when you sand and buff, the results you see are the results you get.
What are you working on?
What type of paint was sprayed?