I just posted this on another thread asking the same question.
I knew it was time when the sound of his key in the lock coming home made my stomach 'drop'.
When you look at him and feel nothing positive at all, it's time.
When you don't want to share a room or sofa with him it's time.
When you avoid anything 'physical' like touching hands, a kiss, a hug, sex etc it's time.
When you find yourself wishing the weekend would pass quickly and you're looking forward to working so you can get away from him, it's time.
When you have to 'psych yourself up' to ask the most basic of questions because you know it may 'annoy' him, it's time.
When you avoid telling him anything good at all because he will suck any joy or happiness from anything you say, it's time.
When you feel the need to put on an 'act' in front of your DC or relatives/friends, it's time.
I could go on and on forever really, but you just need to ask yourself 1 question, Do I want to be in the same position in 1 years time?
You now know whether it's 'time' for you...