For my part, the number is 50+, because that's when I have more than one page of things on Gmail. Since I cleaned it out, I have almost nothing in my inbox except tags, so... It's easy to see when I've been slacking on my tags.
When I reach that point I feel lame for a little while, then pre-write up all my tags in a Notepad file and tag all at once in a firestorm of productivity to empty out my inbox completely!