The only time you should say, "Its been a month since I've mowed" is in January and February. A good rule of green thumb is to never cut off more than a 1/3 of any plant you are trimming. While there are exceptions to this rule, grass is not one of those exceptions. In college, my roommate and I had battles in procrastination when it came to the doing the dishes. We would wait until the plates started to overflow out of the sink, onto the counter, and then finally clean them. It was always a far worse experience than if we cleaned them every night, or even once a week. If this tactic is applied to cutting grass, you will possibly be doing more harm to your grass than good. In the picture above you can see where the neighbor let his grass grow out for several weeks and then mowed it down short. Over time if you let your grass grow too long, the thatch layer will raise, and when you go to cut it where you were cutting it before, you will cut into the thatch layer. Cutting into the thatch layer is bad! It can send the grass into a state of shock, so to speak, and then you're in recovery mode, and depending on the time of year, you could be looking at several weeks of recovery.