How long you wait is all about strategy and making a smart plan. Lots of unusual things can come up. Let's say you get to the start right on time, and a big truck takes off right when you get there. Probably better to wait as long as you can so you're not in their dust... so wait until ANOTHER car shows up in your rear view mirror, and pull in and start just before they get there, giving yourself maximum dust reduction.
Of course, this might change if you think the car behind you will pass you, and you know who they are before they get there.
Of course, this might change if the sun is getting low.
Of course, this might change if it's the second or third stage of the day.
The biggest thing to realize is that every time you get a choice to do anything (like when to check in), that's an opportunity to make a strategically better/smarter decision than your competitors.