Fare rules for long trips
 

I want to buy a R/T ticket CDG-EWR, leaving sept 2nd, but I don't know when I'll be returning yet.
So my plan was to schedule the return for as late as possible (say, january 2009) and just change it when I know on what date I want to travel (and pay the $100 or 200 fee).

A search for this gives me a $1700 fare! Even though a R/T 9/2 to 9/20 is about $700, it goes insanely up if you want to return later.

Does someone know the rules for fares for very long trips? How long do you have to be somewhere for those fares to go up like this, and what's the best thing I can do to solve my problem?

Thanks!

Check the fare rules on the $700 ticket -- discount tickets often have a 30-day max stay (sometimes 60 or 90).

Checking some dates 9/2 - 1/something, there is a $720 all in fare on UA out, Swiss back, with a number of carriers offering < $800 fares (LH, BA, AC, US). Even AA nonstop to/from JFK.

Do you have to fly CO for this one?

Here's the caveat. I want to fly this with my girlfriend.
For me, it's gonna be the trip back from reward travel with CO (I only have miles with CO - leaving 8/17). For my girlfriend, it's the 1st leg of the trip I described, she doesn't know when she's flying back.

So I have to fly CO, or a partner where reward travel is available with my miles (not AA, BA, etc).