For me I had at least 3-5 days of constant, 24 hour contractions. They're mostly irregular, moderately uncomfortable and sometimes slightly to moderately painful. This is my pre-labour and warm up. If I just go about my day and ignore it, it's not so bad at all... but I imagine if I tried to micromanage this phase and time the hell out of it I'd probably find myself exhausted and disappointed very quickly. Following that my contractions would usually very suddenly get intense and closer together, usually over a period of less than an hour. I can't really describe it: they just *felt* inherently different. Often for the first hour or so I'd be questioning it, "Is this really labour? Have they really changed?". I'll take a shower, or lay down, or drink a bunch of water and see if they change... and if the answer is NOPE and they get worse then I know I'm probably in early labour! By that point I'll check my cervix to see if baby is down, I have a bulging bag or my cervix has changed, and usually I find one of the three if not all three at once. ;) The last two times I checked myself in that situation, my hand came out with a big clump of mucous plug - ewww!
At that point I just mill about until I can no longer function normally. I do NOT time contractions and do NOT watch the clock. Once it's clear that I can no longer get up and walk around, even between contractions, then and only then do I call the midwife!