I often say to grad students: a reasonable thing to aim for in grad school is ~2-3 essentially complete and posted papers, alongside your JMP. This often looks like your JMP, plus your 2nd-yr paper, a paper from RA'ing for a faculty, or from working with classmates

Of course, some people get top jobs with only one very, very good paper. This is very risky though - you don't know ex-ante whether your paper will be very good! Most schools care about productivity, so having a couple of completed papers helps

Even if the papers are not change-the-world papers. Also, it exercises your paper-writing muscle. Not everyone comes into grad school ready to write ECMA's!

One "mistake" I sometimes see is some students don't take the 2nd year paper seriously enough. That's basically your only big milestone other than your JMP, so if all the time you put into the 2nd year paper doesn't produce a draft, that's a pretty big time sink

Another mistake, ironically, is taking field class paper requirements too seriously. If you take 3 field classes a quarter, 3 quarters, that's 12 papers! There's no way you can write a different one for each class