With Enterprise, I frequently find myself an hour or so late, and have on very rare occasions been several hours late.

At an hour late, they either don't notice, don't care, or it's just assumed that stuff happens with flights.

At four hours, they whined a little bit and ask about what happened to my flight, but I still got my car at the original rate.

I'm not sure that I would deliberately push it out much further than that, though. Save that for the true flight nightmares.