I just did some testing to establish a baseline for this project. The results are as follows:

Time for running shower to reach comfortable (not full) temperature: 45 seconds

Volume of water used (wasted): 19 cups (1.2 gallons).

Time for shower with head removed to flow 1.2 gallons: 22 seconds.

These results are encouraging. It seems this idea should be able to save more than a gallon per shower and half the time I have to stand naked and shivering with my hand under the cold water. My calculated flow rate is 3.24 gal/min which is reduced to 1.6 gals/min with the shower head.

I would love to hear from any readers what your shower warm up time and volumes are! Testing should be done several hours after the last shower use (first thing in the morning would be great). Run water into a bucket and time how long it takes to reach a temp where you would step into the shower. Then measure the volume. It would also be interesting to hear about your various setups (shower only, tub shower combo, independant or single/mixing valve, etc). My test was done on a shower stall with a mixing valve.