Well, since it's a small matter of programming, it sounds like it'll be done Soon, particularly if you're counting in Valve time.
It's really not easy to estimate the scope of a project particularly when little of the groundwork is done and we don't know the resources that you've got. Are you allowed access to third-party libraries? Do you have a partner or a team? Will you have to spend time drawing your own graphics?
It's also different from person to person - I might get lucky and bang out the combat system in a productive week while locked in my room, and be held up at quest tracking for a month while you've blown through both in half the time.
You should probably try to build a simple system then build content onto it as you go. Start with the combat and if it takes you five months, use the last to turn it into an arena style RPG with a text menu shopping screen between bouts. If you manage to get an overworld map done, use it as a point-A-to-B travel system between multiple combat zones. If you manage to get a dungeon system, you might have enough time to throw in procedurally generated instances or a roguelike.
It's also not a problem to finish early - You can continue to polish a completed system and you may find yourself surprised at the innovations that can come out of it.
The only thing is, we have to write an initial report where we specify the goals of our project. So it will look bad if I implement or do not implement stuff that I have specified/not specified in the project goals.
I am on my own for making this, so it would be the work of just one person.
I was not thinking of implementing a quest system as such, simply something along the lines of go to this location/talk to this person/kill this boss to make the story progress.
I was thinking of making the graphics myself, and using tilemaps with graphics similar to that of Final Fantasy (http://allgamesplayed.com/wp-content/uploads/2010/02/ff1.jpg). But even this could take up a lot of time in the project which could be used for coding.