I haven’t done a full sunset burn-down test with a FW3A, but when I first made the mode on my Brass Beauty, it lasted several months when used every night. I’m guessing FW3A should be similar.
To really estimate it well though, you’d need to instrument a FW3A with a logging DMM and measure power use for the entire hour until it shuts off… then divide the battery capacity by the area under the measured curve.