Hi Mr White,
It’s really hard to say how long when we don’t have your SAC rate. Being new and a little apprehensive about your abilities will increase air consumption. I think your doing the correct thing and staying shallow to get your buoyancy in check. Go down to 15’ and see if you can get your buoyancy stable to where when you take a breath you ascend slightly and when you exhale you descend slightly. This takes practice but when you can achieve it at 15 ft move to a deeper depth and practice there. You should relax and focus on good smooth breathing. Make sure you don’t hit your BC inflation button to long when making adjustments. Once you get a few dives in and become familiar with the equipment things will come easier. Make sure your buddy watches you so they can make recommendations on your trim if there’s an issue there.
Welcome to the forum and the great underwater world.