I've had meltdowns go from really short time, and I mean they're almost over before they actually begin, to a couple that were well past the half hour mark. Always thought they were some kind of extreme anxiety attacks or something.
On the first case, If I walk away soon enough to somewhere I can be alone (restrooms work well), it's enough, 5 mins and I'm back.
The other ones… were real bad. Can count them with one hand, and the embarrassment felt is still etched in my head. Those, I don't know if they can be stopped, or how. They just ride me and then they're over, just creep on me unexpectedly when the pressure is on. In school, at 9-10 years old I had what I remember to be the worst one, spanning almost a whole 1hr class, where I even barfed at one point.
What I've seen, is that as you grow older, they seem to be less often, and not last as long. It's been several years since my last meltdown.