@o-chan:
Not really, the earlier OPs were far more random (how long did we have "Believe"?!) but it did seem Toei had some degree of a time period for the OPs. "We are" mainly covers the East Blue arc leading up to Roguetown and then they changed it to "Believe". "Hikari e" was really badly timed as it spoiled Robin joining the crew almost a dozen episodes before it happened. "Bon Voyage" was mostly a filler OP (meaning that it's just…there). Now "Kokoro no Chizu" was placed perfectly at the beginning of the CP 9 saga and the changeover to HD, as well as the animation upgrade the series received. "Brand New World" was put as soon as they hit Enies Lobby and possibly would have stayed that way IF they did not change the timeslot and as a result we got "Crazy Rainbow" which they retired and gave us "Jungle P" which was place right at the beginning of the Ice Hunter arc.
One thing I have noticed is that Toei has been almost sticking to the changing the OP every 26 episodes format since "Jungle P". While it's not completely accurate they have been generally rotating the openings during the fall and spring anime seasons. The newest one coincided with the Movie 10 tie-in episodes and the release of the Strong World movie. Based off of history we're probably going to get a new opening before the summer is over with and I get a feeling that they may premiere it after the Straw Hat filler is finished.
O-chan
I think its a mistake to look at his in terms of the # of episodes. Its just an arbitrary number that can vary within the negotiated time period for the opening song's usage. In terms of time period though, One Piece has actually been rather reliable with most openings lasting 1 or 1.5 years, usually starting in October:
We are, Bon Voyage, Crazy Rainbow and Jungle P all lasted for about 1 year, with most starting in October. Likewise, Believe, Hikari e and Kokoro no Chizu all lasted roughly 1.5 years. The only big exception is Brand New World, but the reason for why that happened is obvious. Then we have the We Are Remix and Share the World, both of which lasted a shorter time than usually. However these were both done by the same band and chances are they were negotiated/decided at the same time. So if you count those two openings together they last for just a bit more than a year, thus conforming to the norm.
Personally, I think its possible that we'll get a new opening later this summer (but probably not immediately after the Whereabouts episodes). But I think people need to get ready for the strong possibility that we might be stuck with this opening until October.