I definitely think this film will open bigger than any X-Men movie before it. While First Class is regarded highly among we fans, it was the lowest grossing X-Men movie yet ever released and a lot of audiences stayed away at the time due to a lack of Jackman and the horrid taste of TLS and XOW.
While FC brought the nerd community back onboard, it was DOFP that got the GA to take notice again, and I do believe the mediocrity of X3 and XOW still hung unfairly over DOFP and (for that matter) The Wolverine. But I do think DOFP was the "Batman Begins" so to speak for the franchise. The social media chatter for Apocalypse is proof of that. Add on the hyper love people had for Deadpool, X-Men: Apocalypse should quite easily clear $100 million its opening weekend. I wouldn't be surprised if it clears $140 million over the 4-day Memorial Day weekend.
With that said, it doesn't have Jackman, which is a key point with some people (though less than it used to be), and it has stiff competition. So how much it makes after that will depend a lot on the WOM and quality of the picture. If it is as good as FC or DOFP, I could see it ironically having better legs than them. But if it is more average, as the fourth superhero movie in as many months this year, I could see it dropping like a rock. So while a big opening is a definite, the actual quality will play a big factor if it ends up closer to $600 million or $850 million, I suspect.