When I was at Sandhurst (over 10 years ago, mind)there was a married cadet (he had been a soldier who was picked up for officer training) whose wife had a quarter on the grounds, but he had to live in the accommodation during training time (as jcscot said), but was able to see his wife during free time at the weekends etc.
However, having said that it was, at that time, only during the first and part of the second terms that we had to 'sign back in' by midnight. In the third term we didn't have to sign in till the morning, so we were able to stay out overnight which is actually what my now husband and I used to do (my parents lived not far away, so we pretty much 'commuted' when training allowed it).
But as others have said, unless you're married you wont be eligible for a quarter.
Your partner will be expected to live in the Mess when he eventually reaches his regiment for a token amount of time, but that depends on the regiment and the CO at the time. My husband and I actually lived together in a rented house when we were both (newly-engaged) lieutenants, nearly captains (we did have to write formal letters to our COs 'requesting permission to live out'!).
My husband is in a usually pretty traditional cavalry regiment, but his CO at the time was more relaxed so was fine about 1. his getting engaged and 2. living out unmarried, so we were lucky.
Good luck with whatever happens and I hope it works out for you all as well as possible.