Cruise lines will in fact plan their ships itineraries around your charter if your charter is going to areas outside of their normal ports or operation. For example, great opportunities for charters exist when a cruise line repositions a ship after the cruise brochure has been published. Providing the cruise line has not sold an excessive amount of passengers on the cruise date you want to charter, they will "buy" these passengers off. This usually means offering them another date and some incentives by way of soft dollars or whatever. The cost of the buy off will be added to your charter fee, so once the decision to move ahead is made, the sooner you contract the charter, the cheaper it will be. However, there comes a point where the numbers do not work so the cruise line will not move those passengers in order to accommodate a charter.
Royal Caribbean International for example, does not typically consider Full Ship Charter requests within 6 months of the requested sail date and/or for those sailings booked above a pre-determined threshold. Most clients charter one year in advance and some as far out as two or more years.
For Re-Sale charters, it is in the best interest of the client to allow ample time to promote the charter (9 months+). The cost of displacing booked guests will be a factor for any “open” sailing and will generally be lower if the charter is contracted farther out.