I only ever dye my hair maybe once every three or four, sometimes six months. Just because it grows really, really slowly and it's so long that nobody notices an inch or two of regrowth.
As long as you're not pregnant I don't see any reason why you can't dye your hair every 4 weeks or so, try investing in a really good quality set of products to help prolong the life of your colour. Also, cut down on blowdrying/straightening and product use as much as you possibly can, and wash your hair as infrequently as you can. I can get away with washing my hair once or twice a week because I hardly ever (ie, maybe once every two or three months) use product in my hair (and even then, it's just an oil-based shine spray, not harispray or gel that stick and can strip colour), and I never blowdry or straighten my hair.
Also, the best tip I ever got while I was hairdressing is to avoid Pantene products like they're hair-removal creams - they are notorious for stripping colour and they're really no better for your hair than the $3 supermarket brands. Try Redken, I've always had good experiences with their shampoos and conditioners.