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You can go down to seconds, but note that wait steps in the minutes or seconds are likely to take significantly longer than you expect, depending on whether or not there are other campaigns running in the background at the time.
Imagine that your Marketo works kind of like it has a few workers in the office. You tell one of them to go make 300 copies of something. If they're busy that day, they aren't going to just sit there and stare at the copy machine until it's finished - they're going to leave, go do something else, and come back when they're done with the other task to get the copies. Or if someone else is free at the time, they might go pick up the copies when they're done. If everyone else is busy though, the copies could sit there for a little bit.
Wait steps work kind of the same way. When Marketo's "workers" see a wait step, they leave to go do something else, and could get delayed coming back. This is why wait steps tend to be inaccurate on the scale of seconds or a few minutes.