The time depends on each specific context: office, educational degree (undergraduate/graduate), the urgency of hiring, the company, etc. Some examples:
- A company gave me 7 calendar days to decide since they already knew that I was going through interviews with other companies and they needed someone experience to start a project asap.
- During business school (probably the same for undergraduate), the process is more structured, and almost every consulting companies have the same schedule
How to approach the extension? This is a delicate matter.
What I have recommended is that people should approach the other companies they are applying by calling them (if you have the confidence of doing so, otherwise send an email): tell the responsible that you are highly interested in the company because X,Y,Z but that you have received an offer from other company. State that you have a short-deadline to decide whether to accept it or not. Finally, ask if they could please speed up the process
By approaching the company that sent you the offer, you might send the signal that they are not your first choice, leading to a bad perception as a proffesional.
I hope this helps.