I've started a PhD in Psychology and am now considering stopping after completing an MPhil for reasons of time constraint (i'm working full-time along side), but i'm not sure how many studies usually constitute an MPhil. is there any rough guidelines?

You should consult with your supervisor, they will be able to tell you whether you've done enough. If you don't want to speak to them untill you've made your mind up, then you'll need to speak to somebody else in the department in confidence.

If you think you are about half way through your PhD then you probably have enough to write up for an MPhil