For me it's when she starts do get on my nerves or pisses me off, but im on about big style, other ways when you know its time is, never seeing each other or talking, even if its on the phone, you soon find out with SMS saying you're dumped.
For example i've got this girl now and I want to get rid, I dont need a G/F at this point, just going out when you're single is brilliant without fear someone spotting you with your tongue down there throat!