I only blow dry y hair straght once a month. Is this going to bring permnant damage to my hair? I always apply a deep conditioner before.

there exsist these kinda leave-in products that are supposed to condition under heat and protect from blow drying. i never tried it. until recently i used to blow dry (on cool position) my roots cuze i thought it gave me more volume but i gave up doing so cuze the effect doesn't last long enough to even bother.
i'd say, leave your hair airdry until it's just damp and then try the cool position. this way the blow drying is shortened and who knows the effect is similar. but well, i don't think once a month will kill your hair either to be honest  8)

Occasional blow drying on the cool setting should not do extensive damage.  High heat can cause significant damage (split ends and breakage).  Be careful to not brush your hair when it is wet, as that will also cause damage.