The Short Answer
For scarce-inventory properties (Aman, small independent luxury hotels, peak-season flagship openings), book as far ahead as the hotel accepts reservations. For high-inventory branded hotels in major cities, booking closer to the date often yields better rates and upgrade odds, since occupancy pressure is the variable that actually determines both price and upgrade availability.
Generic advice about booking timing treats "hotels" as one market with one optimal booking window. It isn't. A 20-room Aman property and a 400-room branded hotel in a major convention city respond to booking timing in close to opposite ways, and applying one rule to both leads to a worse outcome at whichever one the rule didn't fit.
The scarce-inventory case
Properties with genuinely limited room counts, Aman's roughly 30-room average, small independent luxury hotels, any property during a single high-demand peak window, sell out through simple supply constraint rather than dynamic pricing. For these, the honest answer is to book as far in advance as the property accepts reservations, since availability itself, not price, is the binding constraint. Waiting for a better rate at a property this size usually means waiting for the room to no longer exist as an option.
The high-inventory case
A large branded hotel in a major city, with hundreds of rooms and sophisticated revenue management, behaves differently. As discussed at length in our piece on what "subject to availability" actually means, upgrade odds and, often, rate itself respond to how much unsold inventory the hotel is carrying as the date approaches. Low-occupancy shoulder-season dates and midweek stays specifically improve both price and upgrade odds the closer to the date you book, since the hotel has more visibility into which rooms will otherwise go unsold.
How to tell which case applies
Room count is the simplest proxy: under roughly 100 rooms, particularly at a brand known for scarcity as a deliberate strategy, treat it as the scarce-inventory case and book early. Several hundred rooms at a major branded hotel in a large market, treat it as the high-inventory case and retain more flexibility on timing. A property's typical occupancy for the specific dates in question, information a good advisor should be able to speak to, is a more precise signal than room count alone.
Where preferred-partner timing intersects with this
Preferred-partner amenities don't change based on how far in advance you book, but upgrade priority still depends on the hotel's actual inventory on your dates, discussed at length elsewhere on this Journal. Booking a preferred-partner rate early at a scarce property secures the room itself; booking one closer to the date at a high-inventory property, where the room was never in doubt, can sometimes improve upgrade odds by putting the reservation in front of the hotel at a moment when it has clearer visibility into unsold premium inventory.
The right booking window is a function of scarcity, not of any universal rule about early-bird savings. A traveller applying the same six-months-ahead rule to a 20-room Aman and a 400-room convention hotel is optimising for the wrong variable at one of the two.
The exception: named peak events
Any property, regardless of typical size or occupancy pattern, behaves like the scarce-inventory case during a specific named peak event, a major festival, a marquee sporting event, a destination's single high season week. For these dates specifically, book as early as possible regardless of the property's usual inventory profile.
Our position
Ask what kind of market the specific property sits in before deciding when to book, rather than applying a single rule to every luxury stay. Scarce, small-inventory properties reward early commitment. Large, high-inventory hotels on ordinary dates often reward patience. Confusing the two costs either the room itself or an unnecessary premium.