TITLE: Lesser known examples of perseverance with a successful ending
QUESTION [6 upvotes]: The stories of Wiles, of Perelman, and of Zhang, are very well-known to illustrate that sometimes great results are achieved through particularly long perseverance.

What are lesser known-examples of that kind ?

These need not be epoch-making results like those mentioned, but, say, they required at least 6 or 7 years of dedicated effort (not necessarily by a single person, and also not necessarily a first-time paper i.e. a paper correcting a previously incomplete attempt also would qualify as a good answer). 
Edit: to clarify, I do not have in mind long endeavours by many people over centuries like Galois Theory, really just a particular theorem within the lifetime of their author(s).
P.s. if moderators could help make this question community-wiki, that would be great, thank you.

REPLY [2 votes]: I'm not sure if you're insisting on examples in which a mathematician (or group of mathematicians) works single-mindedly on a single problem for many years and finally conquers the problem.  If so, then the following might not qualify.  But it might qualify if your conditions are not as stringent as that.  I quote from Ilse Fischer's response to being awarded the AMS Robbins Prize.

The idea of working on Robbins' last open conjecture on alternating sign matrices slowly manifested in my mind when I was writing a grant proposal about 10 years ago, when I identified it as an ultimate, albeit unrealistic, goal. In the beginning I hardly dared spend much time on it, but every now and then I discussed it with other combinatorialists. Roger Behrend and Matjaž Konvalinka were obviously among them, but I also had a particularly fruitful exchange with Arvind Ayyer back in 2012, which led us to several conjectures on the enumeration of extreme diagonally and antidiagonally symmetric alternating sign matrices of odd order. About three years later, Arvind, Roger, and I were able to prove these conjectures, and to some extent also this work paved the way for the eventual proof of Robbins' conjecture. I feel deeply honored and moved to receive, together with Matjaž and Roger, the David P. Robbins Prize.