We typically sat in one spot for 45 minutes to an hour and called about every 15 minutes. After souring an area on the same call for series after series over two days, we switched up both the call and the tone and immediately got a response.
Use the wind to your advantage. Like all predators, bears use their keen sense of smell to detect danger, and they'll try to circle downwind of your call to identify your scent. Set up in an area where you can kill them as they try to cut your scent.