We can infer the Irduin chapter will span about a year, though it’s unclear when precisely we’d arrive there.
We leave Sojourn in the early spring, and after Irduin is “A Glorious Season”—implying the [Court] Season in Grand Shayard:
Ciels leans back with a grin. "I heard a tale once about the aristarch of the Norther Rim, who was so enjoying the Season in Grand Shayard that he stayed on a few weeks into the summer.
This suggests that the Season overlaps with spring, suggesting that Irduin would begin in spring and end by spring.
We know per excerpts that we’ll be in Irduin in autumn:
That just leaves the unanswered question of when exactly we arrive in Irduin, given the vast distance we’d need to traverse, along with the implied complications that make returning to the Outer Rim still unfeasible. But it probably won’t take the 5-6 months needed for us to not experience Shayard in the spring Irduin in the summer.