I'm not aware of any changes since then, but the original foundational document on the moderation system says 30 days:
- You must accept the community moderator agreement within 30 days of election or appointment and remain active on the site.
Looking at the most recent moderator election/appointment email I got for pets
We strongly recommend that you read the following to learn the basics of moderating a Stack Exchange community:
....
- A Theory of Moderation. This blog post is one of the earliest references about moderation on Stack Exchange. While some of the details are outdated, the overall feeling still describes moderation on our sites today.
(In theory this is the 'revisited' version, but in this case it doesn't really supersede any of the relevant parts of the answer).
That said, generally the 'soft' expectation is you do so as quickly as humanly possible. If say, you win an election during a vacation when folks are aware you're out of contact — it's potentially understandable that if you don't have internet access (or its limited) you could put it off.