I have an offer from a big 4 and a stat board.

Given the climate, I’m leaning towards the stat board. However, I have accepted the big 4 (since it was offered much earlier than the stat board).

When should I rescind my offer from the big 4?
Due to the volatile period now, I’m afraid that the stat board might rescind their offer as well.

Is there a certain notice I must rescind the offer from the big 4 by? I do not know the starting date yet.

Pls advise thanks