Try to match the habits of the deer when calling. Grunting can be effective anywhere, any time. Many pros carry a grunt tube at all times, and for some it’s the only call they use. Try your call when you see a deer to see what kind of “music” your local deer might like. The key is not to over call.
Usually, soft, subtle grunts do the job, and you can get a feel for what to do by watching a deer’s reaction to your calling.
Blind calling, meaning calling when you don’t see a deer, is another matter. Start with low, sporadic grunts in case there are deer nearby. Increase volume and regularity as time passes.
Also use your grunt call in conjunction with other calls and techniques such as rattling.
Just remember that deer grunt all year, so always carry your grunt call.