OVERCALLING is over rated--Hit em hard and fast. Forget about 20 minutes and make turkeys hunt you. Early on in the season birds are still flocked up (several Hens included)As season progresses then become more passive in you calling---BUT NEVER limit your efforts to a clock, some birds will loose interest.
Easy summation: call as much as needed to keep the gobblers interest focused on you---Worst case senerio is a subordinate 2 year old will sneak in beside you.
Also dont get caught up in switching calls if what you are using is working, but be confident on different types of callers(diaphram, slate and box), each has its place in the turkey woods. Be it windy or rainy conditions, these 3 types of callers will cover most situations. Practice and be confident--NO 2 TURKEYS SOUND THE SAME--they just have kinda the same cadence--get the rythm and keep it real... Best of luck and keep it safe this spring.