Some people on here recommend not negging on online dating sites.
I usually try to put something in about their picture.
The target has already added you as a "friend" so the next step is just to email them.
I possibly went one step too far the other day when negging them about their profile pic (I use this as an opener/breaker) sometimes before immediately going onto other stuff. The neg was about a goofy look.
No reply yet, reckon i should leave it a couple of days and then apologise? I'm sure apologising could be seen as a bit needy.

Actually, you could generalise the question and say when to follow up on a dating site when the initial message doesn't responded to within say 3 days.