We mow according to the growing season which ranges from 28 - 32 weeks on average. The frequency of your mowing totally depends on your preferences
Some people who have a faster growing grass variety, fertilize regularly, or water on a regular basis, etc. want higher frequency in the spring time (5 - 7 days some times depending on rainfall) then spread out to weekly or 7 to 10 days over the summer. Some people prefer a biweekly basis year round. It just depends on how "manicured" you want your lawn to look, the condition of your lawn, the quantity of weeds or crab grass, etc. Please let us know your particular preferences.
We do not mow on a monthly basis because that allows the grass to get too long between mowing, then when you cut it you have a large amount of overlay which can in fact kill the lawn.
We also use a different type of a mower than most services in that ours are "mulching" mowers. They cut the grass clippings up very fine and leave them on the ground. This is recommended by many turf experts because it then returns the nutrients in the grass back to the lawn for further use, basically fertilizing the grass naturally. We can do this with the fine mulching mowers because the clippings are not visible. What this means to you is that we do not have to bag the grass to remove the clippings and your lawn stays healthier year round.
I might also add that we mow at 3.5 - 4 inches on a regular basis. Again, this is the recommended height for this area. Some people like to mow their grass at 2 inches to avoid frequent mowing. However, this is NOT healthy for the lawns so we mow at 3.5 - 4 inches. We use recommendations primarily from Purdue University. They have a excellent website regarding turf management if you are interested in additional information regarding lawn care.