Is this eyeliner smudge proof? How long will it last me without smudging?

It smudges. I have a little bit of oily skin and as soon as you touch it, right off it comes. No matter how long you wear it.

it says its waterproof, does it stay put on the waterline?

I don't use the eyeliner consistently on the waterline, but when I have it does not stay put for very long. But, I also have an issue with watery eyes...so it may work well for you.

Does it wash off easily??

Yes it does. I use Aveeno face wash & it washes off nicely! Blessings!

How does the self-sharpening feature work? Do I have to do something special? My pencil tip is flat now and difficult to use.

They really shouldn't call it self sharpening. It should be called non sharpening, as you don't need to sharpen it just twist to your desired length and use. then twist down when finished. I love this eyeliner as it stays put and does not smudge all day long. No more raccoon eyes!!! I wont use anything else.