I dye mine about every month when the grey wiry hairs start appearing. Although has been 6 weeks as did it just after I had a hair cut and am about to have another one, so want to wait until it is cut before dying it.
I try to dye my hair once a month but sometimes put if off for up to six weeks. My grey seems to be coming through thick and fast since I turned 40 last year. My hair seems to be in 'better' condtion when I've dyed it, doesn't make it dry at all. I use the ten minute ones and love them, I then stand in the shower to rinse my hair. Whole thing done in 20 mins!!