Well to be honest. From my experience. You don't have to rest all the time, in fact you can work out different parts of your body even the next day over.
Chest, obliques, shoulders, all upper body, ect... But if your just talking about abs and just want to mainly focus on them, than...it depends how sore you feel, and how much you actually worked out.
Normally I work out until I get sore. And normally you feel it for ONE to THREE days. But if you still feel like you can work out before then, than nobodies stopping you. The nice thing about abs and obliques are, that there are more than one of them , so you can depend on your stomach not ripping up if you slightly over work out.
But seriously though, do only what you can do. Once you do an awesome workout, and you feel it, rest from one to three days off.
You can work on something in the mean while (Ex. Working on your chest, and than while that's sore you can work on abs and vice versa. But make sure you eventually take a few days rest for both of them, 2 to 4 days typicly.).
Normally the second day is when you'd stop feeling intense pain of soreness. But then is your time for better judgment. If you feel you can take it on, than go for it. Three days is just on the safe side. It really depends on you and how your body responds. But good luck, I hoped this helped you.