Professor Christians at Iowa State Universtiy has this to say about application rates.
HOW TO USE CORN GLUTEN MEAL

"NICK CHRISTIANS
Iowa State University

Corn gluten meal works by inhibiting the root formation of germinating plants. It generally does not inhibit the roots of mature plants or transplants until your reach very high rates (80 pounds/1000 ft2 or higher). It should be applied before germination of the weeds. The weed will germinate and usually forms a shoot, but does not form a root. After germination, a short drying period is needed to kill the plants that have germinated but have not formed a root. Timing is critical. If it is too wet during germination, the plants will recover and form a root. (This is also true of chemical preemergence herbicides).

It is preemergence only, there is no postemergence effect on established weeds. In fact, it makes a great fertilizer for germinated weeds.

If it does not rain in 5 days of application, water it in with approximately .25 inches of water. Then leave a drying period after germination.

It will usually work for about 5 to 6 weeks following germination.

Rates will vary depending on crop and target weed. I generally recommend 20 lbs product per 1000 ft2. This provides about 1 lb of nitrogen per 1000 ft2. Some crops that are grown in rows can be treated in bands in the row and weeds can be tilled between rows. This makes it more economical to use in crop production. Test the material at rates from 10 lbs/1000 ft2 in 10 pound increments to as high as 80 lbs/1000 ft2.

It does not work well with seeded garden crops unless they are seeded deeply (radishes seem to be the exception and there may be others). Transplants or mature plants generally work well. Some producers put down a band, work it into the upper inch of soil, and then put the transplant in the band.

In garden and crop production, growers generally work out their own system, depending on their understanding of the crop they are growing and the weeds they are trying to control.

The material is generally about 10% nitrogen by weight. One hundred pounds has 10 lbs of nitrogen.

The nitrogen will release slowly over a 3 to 4 month period after application."

How long should the CGM be left to dry?

stan, its not really a matter of letting the CGM itself dry. Its a matter of letting the soil dry to discourage weed growth. I would say water deeply once a week for 2 weeks, then go as long as you can without water for the following month or 2. (Water deeply, then let the soil dry as long as you can then repeat.)

If your using CGM for fertilizer purposes only you can water once a week.