LENGHT CREEP
Plotting the beginning time against duration shows how much arcs got longer as the series has progressed.
Length creep could be also partially accredited to the inflated pacing release as well, who see Oda releasing chapter less consistently now than he was used to during the earlier parts of the series.
A crude linear regression using Sagas data points and Arcs data points
It’s quite easy to see that earlier arcs where shorter, on average, and recent arcs are usually longer (and the future ones will realistically surpass the ones we already have). This phenomenon could be called Length Creep and could realistically be defined as:
LENGTH CREEP
A property of shonen comic storytelling, where arcs released later tends to get more long as time goes.
That’s quite common for year-spanning manga series, since you need to overcome in importance every arc with the following one (while maintaining former plot points relevant), thus leading to a slow escalation in their length. Is not a good thing, per se, but it does indeed happen in most of the battle anime manga series out there.
Accounting length creep for arc with different lengths, we have a good fit for a length creep trend for Short Arcs and for Long Arcs (being the ones with the lowest statistical coefficient of variation from the deduced data), that could be used later down the line to extrapolate for future arc length.
While the short arc trend shows a shockingly accurate trend, the one for long arcs is slightly more dispersed, mainly due to the fact that a significative part of the data set for this sample come from the Wano arc (that is still going on). I made all the length creep calculations neglecting Wano, but it’s easy to make them again correcting for a Wano arc once we will know how much long it will be.