Depending on the type of coat that your Griff has, it's grooming requirements will vary.
(Rough coat, Smooth coat) Smooth Griffons - Smooths really require very little grooming. Brushing once a week with a horsehair or similar brush will help encourage healthy skin and your griff will enjoy it. Remember smooths do shed. Coats do vary, even in smooths. Some smooths have very flat tight coats, like a Doberman or Whippet. These will shed very little. Others have more dense coats like a Beagle or Pug and will shed much more. Brushing with a slicker brush and combing can help.
Rough Griffons - These coats can varying vastly from 1 to another. Some are very harsh and grow slowly. These dogs will have shorter sparser beards and leg furnishings. They will never mat and only need clippering, sometimes as infrequently as 2 times a year. The other extreme are dogs that have very dense soft coats. These coats grow quickly. The dogs will have full dense beards and leg hair. They have a tendency to mat, so you must brush there legs and beards weekly or more. These coats will require clippering more often, some as often as every 6 weeks. Coats vary from these extremes. Most typically, you can expect to need to have your griff clippered every 10 - 16 weeks.
Of course, rough Griffs that are to be shown are never clippered. They must be hand stripped.