just from trial and error blind grunting has recked hunts for me unless I know for sure that there are no deer watching me or are to close than a blind grunt every 15 to 30 mins is what i do you never know what you will bring in. I find that if I see the deer a few blows on the grunt tube will usually bring him my way but never over grunt also a good time to grunt is before and after rattling but again never over call another time i found a grunt to work well is during the rut and even the pre rut is to use a doe bleat than a quick grunt call might bring in a dominent buck if he thinks that another buck is tring to romance his does. it dosen't always work but it has work more than once for me always know your surrondings to make sure there are no deer watching you, they can be very very sneaky