Is this standard?
There is no standard. A company or recruiting agency can ask for a response within any time frame they choose. That said, such a short time is pretty unusual, in my experience.
Am I obligated to say yes/no in 24 hours
Of course not.
You can turn down the offer.
You can simply not respond until you are ready to do so.
Or, you could tell the agency that you need more time and propose an alternative ("I really need a bit more time. I can get back to you by the end of the week - would that be ok?")
If you choose not to accept the time frame of their offer, the offer can be withdrawn, but I'm sure you understand that.
, or is this unreasonable?
Reasonableness is in the eye of the beholder.
I would never be pressured into responding before I was ready to do so. But I don't know the reason why they are asking for such a quick response.
It could be that they have another really good candidate ready for an offer and thus need to get a quick decision.
It could be that the company has pressured the agency to get this done quickly for some reason (valid or otherwise).
The agency could feel that pressuring candidates is a good way to conduct business. The company could feel that requiring rapid responses is good business.
You should include several factors in your decision as to how to respond - how good this offer is, how badly you need a job, how likely you are to land other good offers, etc.
If it were me, and I thought this was a very good offer, I'd ask for a few more days. But your situation may not be the same as mine.