I usually ash whenever I start to notice the length of the ash (3-4 times per stick on average). I tend to talk with my hands, so I don't want to fling ashes at people while talking with them.
I ash as least I can...the ash acts as insulation so that the tobacco doesn't over heat...the more you ash the hotter the smoke. Some brands are much better than others where I can go for at least 3 to 4 inches but others can't hold it that long so I pretty much go by what the cigar will allow me.
Unless I feel it's extremely solid, I won't let it get any farther than 2 inches and if I do go farther I sure don't let it rest casually at that point cause I know she's comin down!!