How long before a follow-up

There's no hard and fast rule. It will take as long as they need. It's best to just keep on writing and wait it out, and be thankful you at least got a response to your query. Many don't.

3 months, minimum, if there was no time mentioned by the agent.

As has already been suggested, don't nudge until three months have passed.

Was it a partial or is that a standard part of their submission process? If it's a partial, three months. If it's part of their submission process, it's a bit more complicated. I would see if they are a "no response = no" agency or a "please nudge if you haven't heard after x time" agency. Then follow those guidelines.

Yup, three months unless their site states otherwise. Some agencies say when it's okay to follow up, or what their typical turnaround is, on the submissions page. Twitter might give you a good idea on their timing, too. Quite a few agents tweet where they are as far as queries and subs ("finished everything pre-Oct 9. If you didn't hear back, nudge me" kind of thing).

Unless otherwise informed by guidelines, two months is enough time to wait. This has been the standard for decades.

This isn't a full novel, and it should never take more than two months.