How did Watts possibly have time for all this?
In short: he doesn't have anything better to do, and doing it is a lot better than the alternatives.
As you say, he is in school. He does not need to physically attend school, so he's not wasting time on a bus. He doesn't have a social life with real-world peers; his social circle is limited to online relationships with other Gunters, all of whom are just as obsessed as he is.
He does not have a job, online or off.
He's not inclined to go out and about much; while he mentions scavenging electronics, he also mentions how likely losing electronics to a mugger is, and the stacks are described as slum-like in many ways.
So - school and Gunting are all he does. And as Gunters go, he's an impressive sample. When you only see him with peers like Aech and Art3mis, it doesn't seem obvious, because they're both in the same boat. As Aech points out, he keeps I-R0k around because I-R0k reminds him how clueless most Gunters are. So bear in mind that what Wade is shown doing is well above average levels of engagement.
(Note that Aech and Art3mis are in the same boat - both inclined to "live" in OASIS because [they perceive] their physical selves are at a disadvantage with people, and their Avatars allow them to define themselves in more successful ways. Daito and Shoto as well... all the top tier Gunters we see have reasons to embrace OASIS to the exclusion of the physical world).
So, there's nothing for him in the real world. What can the Hunt offer him?
It offers him a sense of belonging with other people who value what he values, something the real world fails miserably at.
It offers him intellectual challenge that he can't get in the real world.
It offers him the benefits of immersing himself in a subculture, something teens are prone to in any case.
It offers him the dream of immense wealth.
Now, is it reasonable to believe a teenager in these conditions could become a subject matter expert in ~20 years of video games, television, music, and movies? Hell yes. Heck, people did a pretty good job in the actual 80s, even without on-demand access to a vast library of everything, whenever, wherever. As @KevinWorkman points out, this sort of focus is not terribly unusual. And Wade had more freedom, access, and encouragement to focus than most of us do.