It's either a continuous pour or it's not. Whether you have to 'bond' (use rebar/scarify) depends on the thickness of the new pour, the existing substrate and the expected conditions it will be subjected to.
Excerpt from Bonded or Not, a discussion on pavement restoration using concrete overlays, by William D. Palmer Jr.: (concreteconstruction.net)
Bonded overlays become part of the underlying existing pavement—they work together. They are 2 to 5 inches thick and are usually applied to strengthen an existing pavement or for preventive maintenance. Bonded overlays cannot be used if the existing pavement is in poor condition.
Unbonded overlays, 4 to 11 inches thick depending on the desired life span and anticipated traffic, are a pavement structure on their own—the existing pavement is only used as a supporting base. This means they are thicker than an unbonded overlay, but are the only choice if the pavement is in less than good condition.
How long you have until you must take into account other considerations for concrete to self-bond; about two hours maximum. After that, it's way beyond 'begun' to set. Your previous pour should have left some rebar sticking out, if you knew it was going to take multiple days pours.
The longest you can wait is forever (with the caveat of proper surface prep), the longest you should wait is not at all.
