Two different waits, not one
A window project has a manufacturing wait and an installation wait, and they're not the same thing. The manufacturing wait is the time between placing your order and the finished units arriving at the installer's warehouse. The installation wait is simply how long the crew needs on site once the windows are in hand.
Why custom-built windows aren't a same-week product
Because a replacement window is built to the measurements taken for your specific opening, it's a manufactured-to-order product rather than something pulled from inventory. Some manufacturers explicitly separate their standard-size, ready-to-ship products from custom sizing, which is only available by appointment rather than immediate purchase, precisely because custom dimensions take longer to build.[1]
What actually stretches the timeline
A non-standard size or shape, a specialty glass package, or a less common frame material can all push manufacturing time out further than a stock-size vinyl double-hung. If your project includes even one specialty-shaped window alongside standard ones, expect the whole order's timeline to follow the slowest unit unless you specifically ask about splitting the order.
Get a completion date in writing, not a verbal estimate
Washington's Department of Labor & Industries recommends that a contract for this kind of work spell out, at minimum, the price, payment terms, a completion date, permit fees if any apply, the specific scope of work, and the materials and any subcontractors involved.[2]
A completion date in the contract gives you something concrete to point back to if a project starts running long, rather than an open-ended "soon" that never resolves into an actual date.
Scheduling the install itself once materials arrive
Once the windows are on hand, installation for a typical residential job usually runs a single day for a handful of openings and can stretch across several days for a full-house replacement, though weather, unexpected frame damage found mid-job, and crew availability can all push a scheduled date.
How to plan around the wait
Order earlier than you think you need to, especially for anything outside a standard size or a common material. Ask for a written estimate of manufacturing time separate from installation time, and confirm what happens to your schedule if the delivered units arrive damaged or don't match the order, since that can add another full manufacturing cycle to the wait.
Seasonal demand moves the timeline too
Replacement window demand tends to bunch up around spring and fall in much of the country, as homeowners plan projects around mild weather, which can stretch both manufacturing queues and installer scheduling calendars beyond what you'd see in the dead of winter or the height of summer. Asking a contractor directly how booked their crew currently is gives you a more honest answer than any general seasonal rule of thumb.
What a realistic timeline conversation sounds like
A contractor who gives you a single vague answer, like "a few weeks," without separating manufacturing time from installation scheduling is giving you less information than you need to plan around. A more useful answer names an estimated manufacturing window, a separate installation date range once materials arrive, and what could push either one, material backorders, weather, or a change to the scope after measuring.