Three to six months ahead is a reasonable general guideline, giving enough lead time to secure a preferred ship, cabin category, and specific dates rather than settling for whatever's left. Popular ships and the best cabin categories sell out first, especially for winter sailings.
Why winter needs more lead time
If you're traveling during the December–February high season or around a major holiday period, book earlier still — six months or more — since demand peaks exactly when the weather is most comfortable. See When Is the Best Time to Take a Nile Cruise? for the seasonal detail behind that demand.
Last-minute bookings, within a few weeks of travel, are sometimes possible and can occasionally turn up a good rate, but come with real limits on ship and cabin choice — not a reliable strategy if you have specific preferences about the trip.