Unfortunately, there is not one simple answer for your question. The exact days for cheatgrass to go from initial seed development to seed maturity will all vary on soil moisture, environment and temperatures. If your goal is to prevent viable seed from falling to the ground and adding to the seed bank, it would be best to mow within one week of seed head development.

Not all plants will go to seed production on the same day. The longer you wait the more potential some seed will mature. Once you see plants begin to change color (the foliage and awns) you may be getting too late in development to prevent seed maturity. It is also best to mow or weed eating when the plants are still full of moisture to prevent the possibility of starting a fire by removal of the plants.