MH lamps takes about 100hrs for the lamps to burn in. Also depending on what type of ballast you are using makes a big difference. Radium lamps a very particular and require it to be overdriven to get that "radium 20k" look. But overdriven lamps will decrease its life, so would underdriving would also decrease its life, not to mention you would not get the Blue color.
6 month is on the low end. 8-10 month is the norm with these lamps, even while overdriving them.