This question concerns difficulty pronouncing the sounds R and L between the ages of 4 and 6 years old.

Assuming that there's no serious problem (that is, all other sounds are correctly pronounced and speech is mostly intelligible) and that the child will learn to pronounce those letters correctly on their own, how fast is the process of overcoming it?

I don't mean the whole practice and exercises that I've seen advised everywhere, though. I mean, once the child starts pronouncing the difficult letters, is the evolution overnight or does it take several weeks, with some words correctly pronounced and others not so much. Are there moments of backtracking, when they go back to not pronouncing words correctly, or is it more like 'learn the trick and rarely make the mistake again'?

By the way, I'm not from the US and where I live it's considered normal for children up to 6 years old (or older, if their teeth have gaps) to have difficulty pronouncing some letters. Very rarely have I seen or heard of children asked to repeat the word correctly or coached about how to pronounce correctly those difficult sounds, only after they start school. Only if a lot of sounds are not correctly pronounced, making speech unintelligible, will one go to a doctor about it.