I use Wella from Sally-I actually started combining two shades. I also add Grey Magic and use a colorist's brush and comb/pick combo. It really makes a difference to take the time with the brush.
My L'Oreal color treatments *might* have lasted me 4 weeks up north (where my hair didn't grow as fast). Now, I get nearly 6 weeks out of each coloring.
It helps when you take the time and do the brushing and have the proper tools. Also, when you buy your own stuff from Sally, it's a lot cheaper than the kits that don't last!
I'm the lucky one in my family. Until about 5-6 years ago, I only had about 10% gray. Now, it's about 35% and shows up in patches. My Dad was full gray at 25, my mom in her mid 40's and most of my siblings were almost completely gray at my age. (I attribute some of that to the fact that every one of them is/was a heavy smoker and I'm not.)
I'll keep coloring until it goes more than 60% gray. That's what my mom did and it was so attractive when she let it go natural. Granted, she was only 2 years older than me when she did that!
Funny thing is, I had a professional coloring back to my natural color and everyone told me how unnatural it looked! I rhought she'd done an awesome job, but I got asked frequently if I was ill because I looked so pale.
Suzanne