I'm relatively new to digital art, although I am a self taught traditional artist. So far, I've been working in sketchbook pro for about 4 months and just downloaded Inkscape last night (dabbled in it for about an hour).

I'm exploring the idea of freelance work to fund a small indie studio and I've heard that one of the ways to monetize is by charging hourly. So how long is reasonable for character concepts, environments concepts, sprites (2D assets), and eventually 3D assets. I understand that everyone works at their own pace but, no one will higher you if you take too long to get things done, making your prices redicilous.

Are you being supported by your family?
Understand that if you live by yourself, without financial support, you need to charge enough per hour to be able to eat, pay for utilities etc.

With this in mind, have a look at this wiki topic:
http://wiki.polycount.com/wiki/Freelance

Weeeeeeee... one of these questions that are almost impossible to answer.

Apart from what Ashaman said, which gets a heartfelt +1 from me, be reasonable about your skills. You said you started working in sketchbook pro 4 months ago, and that you are selftaught, therefore lack any degree or schooling in art.

I HOPE you had a lot of expierience with creating art prior to your 4 months of creating digital art in Sketchbook pro, because 1) sketching and drawing take even talented individuals years to master, and as there is an abundance of talented people with lots of expierience around, you will have difficulties getting any paid gig without the skills and expieirence, and 2) because of that, you WILL need a portfolio of former work to show... if not hired work, then something you did in your free time for yourself.

That said, why don't you start by frequenting art communities (like deviantart for example), try to connect with other artists, ask them both about the quality of your art, what/how to improve, how to get faster, and maybe how long it would take them to produce a similar result? DeviantArt seems to have evolved into some kind of start-up help for artists that want to go "semi-pro", many game devs seem to look on deviantArt for artists when they have a limited budget for example.

Of course, don't expect to be promoted from "zero to hero" in just months time by doing that. You will need lots of expierience, a great portfolio of prior work and many happy clients before you could even try to play in the big boys leagues. Especially without an arts degree.

Given on how old you are and where you stand in your academic career, maybe switching to pursuing an art degree would be a good choice?

Let me tell you that from my own expierience: Art is both easy to master, if you have talent, and very hard without proper schooling. Being a selftaught artist myself with some drawing classes but no real art degree (I am a Programmer and Engineer by profession, and never had the time and heart to try myself at a second degree in arts... not that it wouldn't interest me), I have no difficulties coming up with good, sometimes maybe even great looking art, as long as it aligns with my preffered style and strengths, and I have enough time.

What I do lack however is expierience in these hard, unforgiving topics that you skip in your freetime, to draw more of what you already can do best. Color theory, efficient coloring, many other things... they are quite alien to me. Yet I often run into limitations to my art and efficiency because of it. Of course you can force yourself to learn this on your own...

An art degree would take that burden off your shoulders, your art teachers would (hopefully) force you to learn what you need to learn to become a great artist.

Of course, you might already be a great artist in traditional media and need neither schooling nor my advices above. In this case, just ignore what I said, besides maybe giving deviantArt a try. Digital arts and traditional art is not that far apart AFAIK, your skills should transfer with little changes.