How long should a sex scene be in a book?

Guess you can tell I'm back to writing. All true, but I'm still reading my 2 - 5 books a week also. Well this novel I'm reading now--Nameless Book--has a threesome that, I swear, takes up 3 chapters. I kid you not. After reading all that rubbing, and kissing and **********and********* for doggone near 30 pages, I was exhausted just like the characters.

And don't get me wrong, it wasn't that the writing wasn't up to par. It was. I just wondered if it was overkill and kinda of pushing the envelope to let the menage-a-trois become the plot versus the action/adventure plot it began with. Here I am ready for the next sex session while forgetting the reason the book was written in the first place.

So, how long (minutes reading or pages) do you think a sex scene should be. Would 30 pages be too long for the same characters? I would love to hear from you.