There is no exact numbers on how long it will take you to become a blockchain developer. If you're self studying then that's good as you've got the initiative on improving your skills on those PLs that are required to be a coder/blockchain developer.
How hard is it to find a job and what is better to choose rust or solidity?
It's hard to find a job if you don't have experience yet. Many companies are looking for someone who's experienced to do the job but, there are a lot of them that tests and give chance to the new ones because it's also an advantage to them because the newer ones are being paid lesser.
Don't think about how hard it will be. If you have honed your skilled on any of that two, the companies will come chase you. Make projects for yourself and use it as your portfolio, posts it in the likes of LinkedIn and you'd definitely see people that needs your expertise soon.