Considering your image says "Recommended length"...I'd guess there isn't one. Most codes for structural steel define certain aspects of the members that have to be checked, either for section strength capacity or peak stress against an allowable, and then have more nuanced checks for connections. But very few are so prescriptive as to define something like this. Minimum 2 bolts, stiffeners at unframed ends, things like that...but this will be based on the stability of the building and strength of the components involved.
I suspect the author of that text has found either though personal experience or industry study that 10% usually ends up being the most economical arrangement for that sort of design.