Depends on the firm, but from my research, usually it is 2 weeks.If you are offered a TC, how long do you have before you have to accept the offer?
But then some firms might be open to giving an extension if you have another scheme etc. but really it is firm dependent. Some MC firms might give you until the end of summer etc. Some US firms require your reply within 1 week. There's no standard answer across the industry.