In my humble opinion, it depends greatly on the woman/nature of your relationship. I've been going out with my girlfriend for over 2 years now.... We met about 3 months before I shipped off to OCS. My OCS experience was 22-23 weeks long (long story). She was incredibly supportive and faithful for every second of it. She sent me pictures, cards, and letters on a weekly basis (after the first couple weeks of course

). She took time off to fly up during the dining outs and see me. She was never needy, and didn't expect anything from me, other than to just write or call when I had a chance (which wasn't often).
Usually every weekend you get an opportunity to make a 5 minute phone call, or get a chance to go out to your car to make a cell phone call. This is the only time I got to talk to her until candio phase, then we talked every day/every other day.
I think it takes a woman that is extremely independant... In other words, her life doesn't revolve around yours. She has her own things she is pursuing and going on in her life to keep her busy. From what I hear, the women that have the capability of being independant (or have their own life) are the ones that make the best military wives.
In my case, my girlfriend helped me through OCS. Getting a letter from her put me in a great mood and took my mind out of the pits. Throughout API and Primary, she helped me study by making flash cards and quizzing me. I honestly contribute a great deal of success (making it through all this) to her support.
On the flip side of this, if you are (or suspect you are) in a toxic relationship, you need get out... NOW. A nasty relationship can have the OPPOSITE effect on your training. Trust me, i've seen it. I've seen a few guys get "Dear John" letters in OCS... I had a friend that was cheated on while in OCS....
The thing about OCS is, that even without all of this extraneous stuff going on, your morale will already be low... You will be exhausted; mentally, physically, and emotionally. If you tack on a relationship going down the tube, or a cheating girlfriend, often times it is enough to push someone over the edge... I've seen 3 DOR's that happened under the stress of a failing/sh*tty relationship.
Think about how stressful it could be and you'll understand why.... Everyday you are getting the crap beat out of you, and you are thinking about her.... What is she doing right now? I haven't talked to her in over a week since we fought... Did she cheat on me? Is she talking to her ex? All it does is build anxiety and desperation... You desperately want to talk to her and 'fix' it... You can't do ANYTHING from the confines of OCS... ZERO contact, and EVERYDAY you are stuck, contemplating it for hours and hours at a time.... You get my point.
Talk to her about OCS, and tell her your concerns. Decide which category your relataionship falls in and act accordingly.