Less! Although you may hear many English speakers use the word lesser, it is actually improper grammar, and not a real word. In some situations you would use the word fewer, but that is only when the thing that is decreasing can be counted and is plural. For example, you could say "I have fewer plates than him," or "There are now fewer hats," but you would say "I have less water," or "I have less time." Hope that helps!

It's a trivial difference most native speakers don't even notice. For some reason "lesser" sounds better to me, but less is probably used more, right or wrong. I learned a great new expression yesterday, even though I'm a native speaker, "never trouble trouble till trouble troubles you." This are one of those little grammatical fine points you can easily get away with just ignoring.

Less would be used here. We use 'lesser' when comparing things especially the importance of one of the things in question. http://sentence.yourdictionary.com/lesser.

Less is the correct one here.