6 weeks is about the cutoff where I would begin to consider adding more yeast. There's probably still enough yeast in there. If you do add more yeast, you don't need a standard pitch rate. Just 1 gram is enough.
I've done it 4 times w/o adding more yeast. The last 3 times, I let the carboy slowly warm up to 60F, bottled it, then let it cool back down again overnight to about 40F. After 3-4 weeks, they're perfect. FWIW, I kept my 1st batch cold the whole time and it took 4 months before I was happy, hardly any carbonation at all until the last 3 weeks which is why I tried the warming trick (which is based on something I found on Wyeast's web site, btw).