i've been straightening my hair occaisionally (at least 2x a month, usually more) for about six years. i bought my chi iron a year ago, and the DRAMATIC difference in the results made me straighten at least 3x a week for a while. i definately see a difference in the health of my hair (in a bad way). it's rougher and i have to use more products. but that probably has something to do with the bleach as well. once i run the chi over it, it looks great, but it's sometimes dry and fuzzy otherwise. i've recently cut back to 1-2x a week. i also don't blow dry my hair. i just let it dry in a low messy bun which keeps the frizzies at bay and cuts back on heat damage.
you can't run 450 degrees over your hair on any sort of regular basis and expect it not to be a little bit damaged or broken. i think "how often is ok" really depends on whether you color treat, how thick your hair is, and how much damage is acceptable to you. for me and my bleached fine hair, any more than 2x a week really starts to show. good luck!