My first character is sitting at level 34, having completed the class story on Balmorra and collected four of my five companions. This means that I'm done with seven of the twelve planets I will visit as part of my class story (not counting Ilum, which is PVP only as far as I'm aware).
There is still solo content left on about half of the planets I have visited, but I have chosen to forge ahead as my level permits in the name of preserving challenge. I'm continuing to tackle content that's 2-3 levels above my head in search of an appropriate difficulty level. In principle, this all means that my next Republic character could have a significant amount of content that I have not seen previously (assuming that I don't go back and power through the old stuff to farm companion affection for legacy unlocks).
Based on this timeline and rate of advancement, I'm probably looking at about six weeks of real time per level 50 character, and about four playthroughs (two per faction, and, coincidentally enough, one for each mirrored class pairing) before I'm really scraping the bottom of the barrel for content I have yet to complete. I have not yet started a single alt due to how the Legacy system rewards work, but to Bioware's credit the only hard part about my next character will be the choice of which of the six classes (excluding Bounty Hunter because it's the mirror of my Trooper) to play next.
Is it ideal for Bioware if I ultimately pay for six months' worth of game time before wandering off into the double-sunset? Perhaps not, but it's hard to call that a real failure either. I suppose I should also be worried about whether all the legacy perks will harm the difficulty level (though I may have gone extra easy on myself this first round by playing a tank-spec with a healing companion). Overall, though, I'd call my first month in the Old Republic a success.