A million.
A million.
That's alright. It depends on height, flexibility, and a bunch of other stuff. I did a 50" last December and went up continually until the box fell at 55" or something like that. Regular vertical jumps where you just got for a certain height without jumping on to a box work just as well and are a heck of a lot safer.
Of note: I had a 25" SVJ and did a box jump of 54" at the absolute highest so just because you can jump onto a high box doesn't mean you can actually jump high.
I have a pathetic SVJ but can jump onto the tailgate of a full-size pickup truck pretty easily. A Tundra, I think. How high is that?