Probably about 3 months. A single application is not all that effective. To quote my friend ScootCat "When someone asks how to deal with crabgrass organically, frequently they are told to apply 20 pounds/1000 feet of corn gluten meal. What you should know is that it takes time and numerous applications of the stuff over a number of seasons for it to even approach the effectiveness of readily available synthetics". I take his word on that.
Bill Hill

Everything I have seen from the research done at Iowa State University on Corn Gluten Meal and how it works and what it does and for how long says that an application of CGM will be effective as a preemergent "weed" control for 5 to 7 weeks.

So do you reapply it every 5-7 weeks? And this will control the crab grass?

"So do you reapply it every 5-7 weeks? And this will control the crab grass?"

To an extent. You need to apply it at a pretty heavy rate for it to be at all effective, and it seems to take several years before it really provides much help.

In my opinion, you're better off working to get your grass to establish and fill in so that it crowds out weeds and you don't need a pre-emergent.