Hi,
No, i don't think there's a such thing as too soon.
from my experience, i think it's important to shadow as soon as you can. i shadowed three doctors, one of which i only spent one hour shadowing before i realized i wasn't going to get much from him. let's face it. your time is precious, but so is the doctor you are shadowing. so, what i'd suggest is finding a doctor and finding out what he/she requires from you to get a LOR. some will say that you shadow for a few weeks. some will say for two whole days. a primary care doctor i shadowed wanted me to learn how to take a history from patients. he was going to write a LOR based on how well i took histories. the prospect of learning to take histories was cool, but i had to study for MCAT at the time. i said to myself, my time is better spent on studying. i also shaodwed an urologist and he wanted me to shadow for two whole days. i made to sure to show i was very engaged and ask good questions.
also, depending on where your live, it can be hard to get a doctor to shadow, so it doesn't hurt to find one as soon as you can. if you stlil have a few years left, i would suggest this. find a doctor and build a long term relationship with him. once it's time to apply to med school, give the doctor two months heads up saying you need a LOR. that way they can schedule it ahead of time. you don't watn to ask last minute. that can be bothersome. the longer you know the doctor, the better letter they can write. good luck!