as for the period to decide on an offer, the standard in the industry is 1-2 weeks; as Hugo mentioned, there could be some differences, but mostly you will receive that time range.
As for the period extension, I will quote one of my previous answers here. I agree with Hugo that it would be safer to speed up the process with the other firms – if you call them and explain that you have an offer and would be very interested to interview with them as well, they will likely accelerate the process. It happened to me after an Oliver Wyman offer with BCG, and manage to complete the whole process in four days instead of the usual 3-4 weeks.
This sometime will also have the additional benefit to decrease the number of interviews you will have to do. Some examples I know first-hand:
- Once I received my offer from Oliver Wyman, I called the other companies I had in pipeline, to mention I had to take a decision soon. I ended completing the process with BCG with 2 interview rounds, instead of the usual 3.
- One of the people I coached went through a first round with a MBB company and passed it. At the end of the interview they asked him if he was interviewing with other MBB and he replied he was indeed in the process with others. When they proposed him the final round day, he tried to put it off in order to have time to interview also with the other MBB. They ended presenting him an offer directly, without even doing the final. Of course, his performance was excellent in the first round, but they would have hardly done so without the candidate making clear he was in high demand.
In case it is not feasible to accelerate the process with the other firms, you have two options:
- Ask the company who presented the offer to wait more – usually difficult to justify and risky in terms of final outcome
- Sign the contract and continue the recruiting process with the other firms – you have to consider if this works for you from an ethical point of view
Hope this helps,