Hope my guild keeps the brakes on for 1 Lich kill.
Farthest I got in Vanilla was Twin Emps. It took months to arrive. All my Naxx experience is from Classic wow gold
Wrath, so I'm not a good judge.
I just know according to my AQ expertise, that as players, what took us months 14 years back took 2 nights to surpass the first week to get a mostly casual father and mother guild of raid loggers. We complete clear in one raid night and I believe that is week ?
I guess Naxx May take about 3 weeks to a month for us to complete clear, and that's an incredibly conservative estimate. I am thinking 2 weeks at most or to at least get to KT in 2 raid nights.
Twin emps/Ouro/Cthun are about the Buy wow classic gold
difficulty average of naxx. Early naxx bosses are going to fall over quick and a few guilds will probably get blocked on after bosses to get somewhat like ours did on twin emps till you repair your strat (or warlocks becoming more SR). Expect average guilds to clean in a few weeks, and several likely all in the first week.
The first couple of weeks, I don't believe my guild will get stuck on any specific boss, but we are very likely to run out of time till we've learned the fights.
Back in the day there weren't many guilds able to attempt 4H. Many who murdered them moved on to clear within a few weeks, but they were also the bosses that motivated people to bother with world fans. Certainly everyone knew going in that they'd take some FrR preparation.