Very few homeowners replace every window in the house at once by choice — budget usually decides the pace. Here's the actual cost math behind bundling versus spreading the job out.
A contractor pricing one mobilization for 15 windows spreads crew setup, travel time, and disposal (a single dumpster or haul-away trip) across all 15 — versus repeating that fixed overhead five separate times if you do three windows a year for five years. Many installers also offer an explicit volume discount once a job crosses roughly 8-10 windows, since it's meaningfully more efficient for their crew's day. The gap is usually modest per window but adds up on a whole-house job.
If budget genuinely doesn't allow the full project at once, prioritizing the worst-performing windows first (drafts you can feel, visible fog between panes, wood rot) captures most of the energy-efficiency and comfort benefit before you've spent the full amount — you don't need to wait years for the payoff on the windows that matter most. The downside is paying that fixed mobilization overhead more than once, and potentially a slightly higher per-window material price if you're buying in smaller batches.
Many homeowners split the difference: replace all windows on one or two problem-facing sides of the house (the side that gets the most weather, or the rooms with the worst drafts) in a single project, then do the rest in a second phase within a year or two — capturing most of the bundling discount on each phase without committing the entire budget at once. Use the calculator with your real count either way to see what each phase actually runs.