How much time would you say is necessary to put down in order to learn from scratch?
There's no upper limit for stopping the learning process (we are all still learning), so it's really hard to give a time frame. However, to make useful contributions, you only need minimal skills (such as editing a text file or following a tutorial), time to experiment, and motivation.
Simply pick a topic that looks interesting to you, and you think is within your reach. Try to solve it; during the process, you will have to look up a few things (maybe some from the above list) or ask some questions. By the time you succeed, you have already learned something new.
If you get stuck with something hard, don't feel bad about it
. Just pick something easier (even fixing a typo or some other minor change will do), then revisit the difficult topic later. At least, that's what I do. In other words, feel free to learn at your own pace, without any time pressure.
The only difficult part is how to keep the motivation levels high. Don't forget to have fun. It's just a hobby project, after all.