It's one minute Per level so you have to use it in minute intervals but you can use it up to your class level in minutes Explained here.
Windcaller (Sp): At 9th level, you can call the winds to obey your commands for 1 minute per level. This functions like control winds, except that you may choose to be immune to any increased wind effects you create. **The ability's duration does not need to be consecutive, but it must be used in 1-minute increments.** This bloodline power replaces thunderbolt.