This is an interesting one. The casual observer probably wouldn't have much time. I give it about 200 to 300 years before most large traces of our civilisations are gone. Buildings, man made structures etcetera. Like old civilisations, our traces wouldn't be more than small ruins. Might be more, but I think that the casual observer would have a hard time deriving our existence after about 300 years.

The second and third questions are a bit more interesting. We as humans have a huge impact on our surroundings. I mean, we domesticated quite a lot of animals and plants. We build houses, we create art and we even use the power of nature itself to do things which our forefathers would have deemed magic. In our inventive years, we have created one material which has changed our way of living. It is cheap and really easy to use and extremely durable, nigh indestructible even. You might have guessed it. Plastic.

Plastic is one of the most influential materials we as humans have ever thought of. It enabled us to cheaply create strong and durable goods. Things like plastic bags or your garden furniture. It also changed the way we think about design. More and more became and still becomes possible because of plastic.

Now, we were talking about traces of us humans if we were to just vanish. I think that the primary thing that would be left if we were to vanish is plastic (in case it wasn't clear). It is so durable that it is most likely that some of it will never be gone. It might be that it will show up in earth layers that geologists watch at when determining the earth's history. However, when that is not the case, we still have the Atlantic garbage patch and it's 2 cousins.

This is not everything tough. Geologists have already discussed the possibility of a new geological epoch because of human activities. I found a short article here about this discussion.