Musicians do not record every note they play, and they play a lot of crappy music while they are learning. The same is true of artists. A lot of bad art is going to be made by you before you start making a lot of really amazing art. This is not just inevitable, it’s also mandatory. You need to make a lot of mistakes, to see what doesn’t work, before you narrow in on what does work for you.
Work fast and cheap when practicing.
If every piece you do is a multi-day effort where you attempt to polish it to perfection, you may be putting in the hours, but you aren’t putting them where they need to be spent. It is better to do 20 images than 1 when tackling new concepts. That doesn’t mean to work sloppy, or with excessive speed. You still want to strive for precision, it just means get the work to a certain level, just where it needs to be for what you are trying to work out, then move on to another. You will learn more doing 20 sketches of hands, than doing 1 sketch that you erase and refine, and shade to perfection.
This is equal parts practicality and psychology. The practical is, there is no need to use expensive paper or top quality paints to work out problem solving. You’ll be doing lots and lots of studies, making mistakes, and generally not making stunning art, so why spend a lot of money to do so? For drawing, simple newsprint or copy paper is just fine. When working in color, you can use inexpensive paint and material as cheap as cardboard to do quick throw away practice work. Also, assuming you have a computer, there are a variety of both free and paid for software packages that are great, and once you have them and are set up, you can do endless studies and never have to worry about running out of paint or material to paint on!
The psychological reason to work cheap? To overcome the mental expectation that the clean beautiful paper needs to contain something “good.” Have you ever done a piece of artwork on the back of a sketchbook, or an envelope or scrap of cardboard that you really liked, but froze up with a nice large sheet of brand new paper? It’s because that scrap drawing had no expectations. It was just pure freedom. If you “messed it up” no big deal, you were going to throw it away anyways. This is the same concept when working with cheaper materials for practicing. You want that freedom to make mistakes without any guilt.
So, when DO you want to spend a little more time and money? Every so often it's going to be great to really sink in some time and see where all that practice is taking you. This is the pay off for all that hard work, and a great time to see where lessons are really becoming second nature, or see areas where you still need to progress. By all means, do some awesome work and spend as much time as it takes to see what you are capable of. Do some pieces that you can very proudly show off, but remember to spend more time over all nailing those basic concepts and lessons, and don't worry that you may not have a ton of portfolio pieces right away.