At What Point Do You Consider A Collection Done?

I'm sure the majority of us would not recover the money we spent on our coin collection . But that's not the point ( to make money ) for me and many others to just take pride in our accomplishment and to have fun collecting .
As far as I'm concerned the 16-D in my Mercury dime collection in mid to upper circulated condition is finished without it , and I'm ok with that . Same for my Walking Half set, all nice circs but no 21 or 21D . And most of all my Lincoln wheat set 1909-1940 pds In VF-20-AU-58 complete except for the S-VDB . And I'm ok with that too .

As a collector I pursue a series until I think I am done and I consider it done.

Me too, I hate paying the full price, but its cool when all you have is just one hole and saving up for it :)

I've seen a lot of people here recommend this, though usually for slightly different reasons - basically, key dates tend to get more expensive faster than more common dates, so (supposedly) if you don't buy the keys immediately, it would be much harder for you to afford them later.

Of course this applies much more to the big-name US coin series, where the key dates are expensive but otherwise easily available; if you're collecting something else, you might end up having to wait months or years until you find the rarer dates/types anywhere anyway, so you might as well stock up on some commons while waiting for the key.

I also would say that "start with the keys" is very bad advice for YNs (and/or low-budget collectors in general), who often wouldn't be able to afford the keys at all until/unless they start earning more money; some of them might even end up sticking with a key-less set, because those few pesky remaining holes just don't feel worthy of spending that much money on.