If I've heard right, a comedy should be 110 pages. How long should the middle and end be?

Comedies have the exact same structure as anything else.  Most films are p1-12 intro.  p12 is some kind of catalyst.  P28-30 between here is plot point one.  At two is 30-90 and can be split into 30-60 and 60-90 eah segment increasing dramatially.  p90 or so is plot point 2 and then we have act 3.  Act three is a wrapping up and in comedies is usually really short.  Comedies are usually between 90-110 pages - but then again that's the trend for practically all genres nowadays.

The standard Spec Script should be 90 pages and 100 pages at the most and this goes for any genre - Comedy included.

Personally, my advice is stick to 90 pages if you can, it's preferred..

From a brass-tacks structural standpoint in, say, a ninety page script:

Act 1 - pages 1-20
Act 2 - pages 21-70 -- mid-point around page 45
Act 3 - pages 71-90

But the best thing to do is learn that structure, internalize it...then don't think about it as you write.  Let your story flow out of you, staying aware of what is needed and what isn't needed to propel the story.  

But do try to keep it around 90 or so pages, as mentioned, and no more than 110.  There is such a thing as too much of a good thing and depending on the kind of comedy you're going for (slap-stick, word-play, subversive, etc.) too much more and the audience will likely go numb from it.  Look at successful comedies and they're running times -- equate the number of minutes to an equal number of pages (roughly, one page = one minute of screen time).

But most importantly, if you're writing a comedy -- make sure it's funny.  Let the humor flow from the characters and the situations, and don't foce it.  If you force the humor, it becomes LESS funny.

Keep writing, and good luck!

- Lon