That depends upon the battery model (type) and its associated capacity. For example, Eveready's Energizer e2 (model X22) is rated at 655mAh, while the standard Energizer alkaline (model 522) is rated at 625mAh. Their carbon-zinc equivalent (model 1222) is rated at 400mAh, and their NiMH rechargeable equivalent (model NH22) is rated at only 150mAh.
BTW -- according to the datasheet for the X22, approximate service life at 10mA constant current drain is listed below for the plotted cutoff voltages:
10mA draw to 7.0V cutoff --> approximately 45 hours
10mA draw to 6.0V cutoff --> approximately 58 hours
10mA draw to 4.8V cutoff --> approximately 65 hours
Eveready rates their battery products in this category to a cutoff voltage of 0.8V/cell, meaning 4.8V for the 9V models.