First, Welcome.
We are left to making assumptions due to the lack of info provided. A rig description in your signature and/or profile would aid contributors in providing accurate info/suggestions based on your real world situation.
- Your FW Tank has a finite limit. More flow = shorter usage.
- There should be at least 1 restriction in your shower, possibly 2. These are intended to limit flow to conserve your resources.
- An Oxegenitics shower head will give better performance and reduce consumption even further provided the OEM one is a different brand/model.
Removing the shower hose from the fixture should reveal the primary restriction.