Lengthy things are long and drawn out. Your teacher's lengthy lecture might extend far beyond the end of class and leave many people dozing at their desks.
The adjective lengthy is almost always used to describe a duration of time (or sometimes a long piece of writing), rather than the physical length of something. So long-lasting speeches and endless waits at the bus stop could both be called lengthy, but you wouldn't describe your friend's long hair that way. Lengthy is an 18th century American invention, also adopted by the British in the nineteenth century.