Often I’m asked “Do you change your flash batteries when you shoot all day?” 99% of the time the answer is no, and it has to do with the kind of batteries I use as well as the power settings I use on my flashes.
Secondly, I do not shoot my flash in TTL mode. MOST of the time my power is set at 1/8th (maybe 1/4) and I set my camera to make the exposure work based on the flash setting. Granted, in bright sunlight this won’t work (you need more power outside), but indoors there’s a very good chance 1/8th is all you need if you are using a good DSLR and can put your ISO at 1600 or higher. This past weekend I worked for 12 hours and didn’t have to change the batteries in my flash once. Don’t get in the habit of shooting TTL! It’ll drain the batteries in your flash really quickly! Learn how to manually set the power on your flash!