If you practice a lot, there's a higher chance you'll develop the muscle memory to type quickly and accurately. At the same time, perfect practice makes perfect. If you practice for an hour doing the wrong thing, you'll keep doing that wrong thing.
I recommend you practice for 30 minutes at a time. That way you won't hurt your wrists by being on the computer typing for an hour or two, and you'll be able to more easily make you maintain proper typing form.
I practice a lot on this website, and it can be very satisfying to do for a long time, but after a while it can become frustrating, so a short session will avoid this. Additionally, I probably should've known that practicing for a long time can lead to typing with incorrect form. For some odd reason I press the "y" key with my right hand, when in reality you should press it with the left hand. It's just one of those things that happens when you practice the wrong way for a long time.