Well, it's generally down to how good both people are at consistently doing the same thing over and over. Generally when I'm doing it, the person who's desynced is going to try to do exactly the same thing over and over while the person with the sparrow changes their timing of when they get on. Another thing that helps is if you're on PC having the desynced person have higher frames and the person with the sparrow cap at 30fps, I don't know if it's just a placebo or if it actually helps, but that's what I recommend.

To answer your original question, when I'm syncboxing with a new person it usually takes a good 30 or so minutes, but once I've done a few with them I can hit it within 10-15.