I would like to know where to stop formwork for beam for slab. There are multiple spans of slab and divided by construction joint. Is that 1/3 or 2/3 of length of beam?

Please clarify your question. Are you asking about the size of the slab segments that are to be joined together via construction joints? It really has nothing to do with the length of the supporting beam. Talk to the structural engineer to see how construction joints should be implemented and what type of load transfer devices should be used. As an aside: although you didn't ask about contaction joints, you would typically place them about every 15-20 feet for a floor slab.

Sorry. I also think my question really confusing. Let me ask again.

Let say I have a slab which surround it with 4 beams and columns at corners, and I will stop casting concrete slab at those beams too. Since I have other span of slab which will connect and cast later to this span, do my beams should stop cast at column or further and if yes then how long should I cast the beams after edge of column?