I'm not sure that anyone could answer this with a specific number of days or weeks. What I do is leave a small "spill" of about 6" in diameter by the end of my walk. I travel this walk daily and when the CGM pellets disappear, I put down another application. I apply it for crabgrass so I put CGM down twice yearly - the first with the earliest of the forsythia bloom and the next when the pellets from the first application disappear - and my intent is to avoid any "break" in the CGM barrier. You are likely dealing with other weeds which would dictate their own application schedule but it is rainfall that has the leading role with how long an application lasts. Incidentally, I try to put my first application of the year down just before the rains (otherwise I'd have to water it in myself).

Don't waste your money on CGM. As a pre-emergence it is not very effective, in fact I do not believe it does anything. As a fertilizer it is excellent, but the most expensive fertilizer money can buy. Example you can buy a 50 bag of CGM for around $60 to $70. It takes 10 pounds of CDM per 1000/ft2. That works out to $60/5 = $12 unit cost (1000/ft2).

On the other hand 50 pounds of soy bean meal cost around $13 in my area, and you use 14 pounds per 1000/ft2. That works out to $13/3.57 = $3.64 unit cost.

So what do you want to pay? $3.64 or $12?

In watching this forum over the years the best prices I've seen for CGM always come from Western Ohio or Indiana. I've seen it for $8/100 pounds. That is an incredible value, but you don't ever see it that cheap in Texas.

As to your question, I don't think the answer has really been determined. As TW said, it is not very effective - at least not for me. I've used it off and on over the years and never noticed any suppression of weed seeds. I'm trying it again this year on a monthly basis. Don't expect it to work as anything but a fertilizer.

I've seen it for $8/100 pounds. That is an incredible value, but you don't ever see it that cheap in Texas.

You can find it in TX for a lot less than that at $170/ton delivered in bulk form. However it is the very fine grind like powder and bought in bulk. You cannot use the powder form. It needs to be pelletized, and when you do that crosses the line to organic fertilizer and ISU wants its money. Its a racket IMHO

I bought my CGM from csn stores in January. 60 lbs for 80 dollars delivered, free shipping, no tax. But they are out of stock. It is recommended to put down twice a year and it takes a few years to work. I put it down last month and so far no crabgrass, very few dollar weeds and grass looks great.

Wow,,.CGM 8 bucks for 100 lbs...amazing....we can't get it in FL.

TW that is feed lot prices. We get the same stuff here. It is as fine as dust.

totsuka, "takes a few years to work?" Where did you read that? Once the microbes get to it the stuff is gone in 3 weeks. There is no build up of anything that I'm aware of.

To the question, I am using it currently ever month. I don't expect anything to happen but that's what I'm doing.

Here is a picture of a pile of CGM at the feedlot outside Wheeler, Texas

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It was sundown so the colors aren't quite right, but that big pile is all CGM.