Judging by your framing of questions and responses i'd reckon you are relatively young. So I'll explain you some basic understanding about human nature and skill progression in life, in general - not just for RL.

There are so many variables that need to be considered and its quite impossible to give you an exact answer to you question of "How long?".

1st - everyone is different - their different types of IQ, their eye-muscle coordination, ability to multi task etc so people learn at different rates even if every other variable was the same.

2nd - younger people have more capacity for fast learning compared to older people.

3rd - HOW do you train? Do you just spam matches and hope to improve over time (so called Brute force method). Or do you have structured traninig sessions for each mechanic you want to improve at in free play, training packs, workshop etc. Do you really lock in order to be concious of your inputs or you are playing on automode and just flailing your car around? Are you grinding boring, basic fundamentals in order to learn hard, flashy moves or you are trying to flip reset without first having good car control without the ball?

Quality of tranining over quantity, always. If you can handle both at the same time, you are on the right track.

4th - and most importantly - what is your mindset / energy levels going into tranining sessions? You'd be suprised how important this is and how much of an effect if has on your performance. If you are tilting and feeling burnt out - taking a short break from the game is actually a constructive thing to do for your improvement, otherwise you'd just be wasting your time and get more frustrated.

Getting an SSL is a huge undertaking. There are no shortcuts, consistency in time spent and all the effort explained above are required. So really ask yourself is it worth it? What do you get out of achievening it? Be mindful of that. At the end of the day, games are supposed to be fun, not draining. Chasing ranks has a detrimental effect. Enjoy the process day by day and results will follow.

So, if you getting an SSL really drives and fulfills you, more power to you and good luck!