Well mine is still basically brand new even though I bought it for months, I only used it like 20 hours or so on Zelda...
I never entered RCM yet, but I also have the impression that one joy-con vibrates more than the other, can remember which one and I just left them in the console all the time, never even played wirelessly other than 4 testing only...
It could be normal too, no idea, because one side is a bit different from the other inside and something might be dampening the vibration more than the other.