Tech school is just over 3 months long for 3C0, but thats just one of several fields in C-CS. 3C3's for example, only go to tech school for about 5 weeks I believe.
If you are considering ANG or AFR, I'd recommend speaking to a recruiter about what 3C positions are available at bases near you before picking the one you really want. For example, the 917th Wing (Barkdale AFB Reserve unit) has only 3C0X1 AFSC's in our comm flight, there are no reserve 3C0X2's or 3C2's.