gehardl pretty much nailed it as far as I can tell.
The amount of variables is ridiculous for something like this. Spray pattern, droplet size, mineralization on the shower head, bath/shower surface area, drain rate, room temperature, room humidity, bath/shower surface temperature, shower head height, shower head angle, water pressure, room airflow, curtain size, curtain material, bath/shower surface material, bath/shower surface thickness, body size, hair length, hair thickness, hair quantity, skin saturation, evaporation rate, temperature, how one showers (ie. if you fling water around while being wetted or stand there unmoving), etc., etc., the list goes on and on...
If you actually need to know this information for some reason, directly measure your flow rate & temperature in and directly measure your flow rate & temperature out.
Andrew H.