How Often Should I Ash A Cigar?
Overashing can lead to an uneven burn. Oxygen feeds fire. The thicker the ash, the less oxygen reaches the burning portion of the cigar, and the slower it burns. While too much ash can cause a cigar to go out more quickly, too little ash can cause it to burn too quickly and, often, unevenly.
And with no ash to protect it, you may inadvertently knock off the entire burning portion, a.k.a. the "cherry," forcing you to re-light.
Ideally, try to keep the end of your cigar covered with a thin band of ash. It will protect the cherry and regulate the burn.
For more, check out How To Ash A Cigar.