As stated below, it is all negotiable.
If your agent is asking for a longer period of time than you are comfortable with, then it is likely that you might want to keep looking for another agent, one that you feel confident will see you through the entire process; in today's market, with limited inventory, this can be a more lengthy time frame than in days when inventory is plentiful. This is a negotiable item, but it is important that both parties feel comfortable with this issue.
The holdover period is, in my opinion, fair. It simply protects the broker, making sure that they get paid if they are, indeed, the procuring cause for your purchase. Somewhere between 30 and 90 days is fair. And there is a clause that, if checked, protects the buyer from paying the agent a commission, even if in the holdover period, if another agent is getting commission for this transaction.
Best of luck.
Your Castle Real Estate