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I sent the screen shot that shows the dates of all of them, and about 1 per week...even that was alot :)
 
I didn't send any content of the messages, that would be to much an invasion of privacy.
I sent a log from all the Skype calls we did, that was already enough for 7 full pages (in a small font size).
 
i sent one day's worth of messages from a "random" day at the beginning, one day from in the middle and one day from near the date the application was sent... i picked days that demonstrated things like - asking a lot of exploritory questions for the beginning, - developing feelings appreciation and distress over distance/separation for the middle, and a disagreement we had over the application (and a bit of the making up :) ) for near the "end"... i did this for just about every aspect of our relationship proof with the constant disclaimer that this was a sampling of our "xyz proof" that demonstrates our relationship progression and strength, and that other samples or the entire history can be produced on request.

in entirety, our application was under 50 pages and we were approved very quickly for our visa office... the trick isn't necessarily in how much you present, but often more in the organization and presentation... make the VOs job an easy one and they may very well return the favour!

of course there are some offices that are known to be more documentation obsessive, so knowing the norms/expectations of your office also helps too!!

good luck!