I usually only watch 4 eps a day of whatever show I'm watching, and even with a schedule like that I manage to completely finishing rewatching shows in a month or less. Only a few shows I watch have 100 episodes or more, and even then it doesn't take much more than 2-3 months to rewatch it.
Back in 2015 I rewatched all of Dragonball/Dragonball Z and the movies/specials to prepare myself for Super and I did it all in about 4 months I think. And that's like 400+ something episodes.