You don't indicate if you are a new owner or iff this is a new occurrence. There are a number of threads that have addressed this condition. First, you don't have to turn on for 30 minutes...just run hot water in the lavatory for about 3-5 minutes and this will get the AH diesel (or propane) burner going. I've found that if you don't set your hot water adjustment to the hottest point on the handle, the hot water won't last. Then the next easiest is to pull out the scald control in the shower handle (to get to it, use an Allen wrench on the bottom of the handle to open the handle up; you should then see a red scald or click it to the left counterclockwise; some have removed it). The next easiest is to adjust the mixing valve to a warmer setting...see the AH manual for Mixing Valve.
To offer more, need to know propane or diesel. Could be a pump issue.
Dave and Beth
2015 Cornerstone 45J
2020 Ford F-150 Lariat