So to be honest, there isn’t really any “normal” grace period. Each company you talk to/get an offer from is going to have their own idea about what is an appropriate amount of time to keep the offer available. The best thing that you can do is be open with a company about your overall interview process, and give them a timeframe. The worst thing you can do, is just perpetually ask for more time. The alternative, is to decide ahead of time just how long you’re going to need to talk to this other firm. Once you’ve got an idea of that, you can talk to whoever extended the offer to you, and let them know that while you’re really interested, you were hoping to finish out your other interviews before making a final decision. Then let them know what that decision date/timeframe is. They’ll be able to tell you if they can accommodate, or how long they’re willing to wait. An added level to this, is to ask to use the extra time to meet more of your prospective team members, get a more thorough walkthrough of the offer details (equity breakdown, benefits explanation, etc.) Those extra steps can help make clear to the company that you’re still very interested, even though you want to finish up other interviews.
One other thing though! Do the same with this more enticing firm. Let them know that you have other offers but you really want to interview fully with them before having to decide on the other offer(s). This can help speed up their regular interview process if they’re genuinely interested in you, and hopefully limit the amount of time you have to keep the other company waiting.