How important are the years of commitment in extracurriculars?

If an E.C.'s expresses your talent, interests, Spike(s) then you should do it as long as possible. For instance if one loves playing the Oboe, is a great Varsity Tennis player, and loves being in student government, then doing it for 4 years fully supports that narrative and application readers love that. However, if one is more of a follower versus a leader, they should stop for a moment and decide which ECs support their story. If one is involved in 10 ECs like 2 sports, 2 music, 4 STEM clubs, 2 volunteer groups, now that's too many. Figure out which ones have the most leadership potential and which E.C.'s one is actually good at. Drop all the other ones fast. No one gets rewarded for doing 10 random ECs that have little to do with one's main Spike or narrative. In fact, if one just list 10 activities one has done over 4 years where they have no leadership, no awards, no personal growth, then they may pre-judge someone like that as having no direction. The key thing is determining what the 1 or 2 main talents are and then picking ECs that support that spike so by the time one applies their spike is 10 feet tall instead of 2 feet tall. Sorry for being blunt and direct. Remember, most top colleges review applications holistically so it's easier for the application reader to recommend, advocate, or admit someone if all their criterion lines up. For instance in the first example if that student played Oboe in the State Orchestra, won 2nd in State Tennis, and was ASB President in his HS, then those 3 activities are more meaningful than 10 ECs where there isn't a lot of depth. Hope that helps.

So all 4 years is definitely not at all necessary but if you are involved in activities since 10 you should be set. I only joined one club freshman year and I anticipate no difficulties but it was a semi-demanding club - debate. But assuming you have meaningful ECs with some leadership you should be competitive for ivies but ivies are gonna ivy and have a sub 10% acceptance rate.