I can categorically confirm that some of the worst upsets I've experienced occurred without any warning whatsoever. Most of these were wake vortex encounters (don't you just love the accuracy of modern altimeters?), some were the edges of jetstreams and the rest were mountain wave. I've also had a few surprises with Cbs where there was no turbulence whatsoever and even bigger surprises with very small Cu where my eyeball's nearly dropped out. But generally, there is some warning which is why the seat belt signs are normally on in anticipation.
PM