This is one of those 'it depends' questions (like most important ones are!). I've quit on the spot once and I think it was ok. I was consulting (paid hourly),

This is one of those 'it depends' questions (like most important ones are!). I've quit on the spot once and I think it was ok. I was consulting (paid hourly), had multiple disagreements with my 'boss' (CEO) about major work behaviors (wanted to literally sit behind me while working and watch, critiqued my work despite not being technical and tried to control everything). No loss, had plenty of work elsewhere, wasn't going to use him as a reference. Other times I've quit from places I've worked at for > 6 months I gave at least 2 weeks notice. So, use your judgment :)