A Geeky Viewpoint

Domestically the opening weekend should be in the 75 to 95 million Range, with a guess that 89 is about right.  

In the past three years we don't have true comparable, based upon schedule and genre.   Last year Iron Man 3 dominated the market, but it was one of 2 Sci-Fi movies for over a month.
  
With Godzilla opening the weekend before and Amazing Spiderman 2 four weeks prior there is a chance of a little Sci-Fi burnout taking place.  My guess is its Godzilla who takes it in the shorts in terms of attendance.

The last two movies in the series have been low in terms of turnout, and their numbers have been in the mid $50s.   The reviews of both were slightly positive but with a negative undertone.     The trailers for the last two movies underperformed, while X-Men Origins: Wolverine had very well received trailers and was extremely disliked by audiences. 

Domestically I am guessing, based on comparable films that low $200 would be acceptable, the under over about $219.

Internationally the X-Men series has been better received, guessing that 55 to 60% of the receipts will be international markets.  That suggests in the neighborhood of $585 Million.

Edit - April 24
In looking at several other predictions its clear that the other predictions are giving X-Men: Days of Future Past an "Avengers Bump" that I am not giving out.   In my mind the "Avengers Bump" is applied via of the comparable Super-Hero movies in my formula, however for clarity if I use a true "Avengers Bump" the numbers become $130/150 opening, $360 Total Domestic and $1.05 Billion International.   I like many other predictors don't think ticket sales that high are out of the question just not very likely (maybe 25% chance.)    We should know more where the Late Hype meter is in about two weeks.