Looking at what is going on in World of Warcraft at the moment, I can only conclude that today's release date for Rift has been perfectly chosen. With the level 80 to 85 content of Cataclysm being on the short side, and the "flow" from reaching level 85 via dungeons, heroics, to raids being somewhat bumpy for a large population of WoW players, the time is ripe for a game like Rift to tap into this pool of frustrated and/or bored World of Warcraft players eager to try something new.
Having said that, I must nevertheless remark that other MMORPGs started strong as well, only to decline quickly afterwards. No, I have no reason to *predict* such a fate for Rift, but on the other side I also cannot exclude it. Which brings up the question at what point in time it is safe to say whether Rift is a smash hit or failed to live up to initial expectations.
Most certainly the honeymoon period of any subscription MMORPG is at least one month long. That is simply due to the business model which packs one free month with the box. Even if players actually stop before that time, the game company would still count them as "subscribers". Going from "playing a game from a box" to actually paying a subscription is a significant step, which gives some players reason to pause and consider whether they actually want to continue playing. On the other end of the spectrum I don't think you need years and years to say something about the success of a MMORPG. Sorry, WoW-Haters, but in whatever terms you describe your projections of WoW's future doom, it is too late to erase the impression of "most successful MMORPG ever" after over 6 years.
Thus I'm falling back on the wisdom derived from when two people who tend to strongly disagree with each other about everything manage to agree on something. Syncaine and me once bet whether Blizzard's next game would or wouldn't be a success, and we set a criteria for measurement of over 1 million subscribers after 6 months. I'm not holding Rift to the million subscribers number, but I do think that the 6-month period was wisely chosen. After 6 months player tend to know whether this is a game they want to stick with, and the period is long enough to make possible hidden flaws with e.g. the endgame apparent. So let's reconvene here on September 1st and discuss whether Rift was the game of 2011, or whether it tragically petered out after a few months.
What do you think? Is six months a good evaluation period for a MMORPG?