I have an '07 1200C. The low fuel light starts to blink at about 100 miles, one steady about 115. I've only pushed as far as 120 since it's fuel injected with no petcock and no reserve.
A lot of it depends on your riding style and whether or not you're riding 2-up or solo. I average 145 - 160 miles before I fill riding solo, but when the wife is with me, I start looking for a fuel station at the 110 - 120 mark. She's petite, but the extra drag of a second person on the bike does affect fuel economy.
I have an 06 1200 custom as well and I usually just fill up around 140-150 miles range. I usually dont even dip into the reserve unless they were hard miles! Although I dont stop putting fuel in the tank when it the nossle clicks off, I fill it to the brim!