About 18 months ahead, sometimes closer to two years. That means a season two years away is often already on sale while the one in front of it is filling up. The useful quirk is that the far-out season sometimes carries better early bird pricing than the nearer one, because the near season is already selling well and the operator has no reason to discount it.
Demand is currently strong enough that departures sell out two to three months ahead, which is why operators hold their pricing rather than cutting it late.
If your dates are flexible by a year, it's worth asking us to compare the two seasons side by side. People assume the nearer season must be cheaper because it's closer to departure. That isn't reliably true any more.