If you put daily effort into it and looked at the right tutorials then you could have something like this in about 6 months to a year. Again, you need to put a lot of effort into this, keep trying and trying.

A FPS is pretty easy to set up and I could have all of that done in about an hour. Setting up AI is a lot harder though, but there are tuts on how to work this and there is an AI pack already out that could handle this. If you want to skip that and do only MP, then it would be just as hard, again though there are lots of tuts on this as well.

My advice is to hit every tutorial that you can find for about 3 months. Daily work, then start on your game and give a few months to have something. In the end you get out what you put in.