How high can the shininess shader data setting go?

There is information about the specular map here. And you can use download a special blend file that will help you pick a good specular color here.

If you're relying on the color of your specular map for the most intense shine, the specular picker blend file is the way to go. And it's easier to make a DDS file. A PNG relies on transparency of the PNG, itself, to create shine and this is hard to get right.

If you're talking about some sort of entry in the warehouse, you could find something in the game that has the shine you want and study how it's set up, or you can just plain experiment with different numbers. That's a pain but most of us end up experimenting at some point.