Replacing 20 windows typically takes 2 to 4 days for a professional crew, though the exact timeline depends on several factors.
A standard-size window replacement usually takes 30 to 60 minutes per window once work begins, but you also need to account for setup, cleanup, and any unexpected issues like rotted framing or custom sizing.
Factors that can extend the timeline include: window style (double-hung windows install faster than casement or bay windows), whether you’re doing a full-frame replacement versus an insert replacement, house siding material, weather conditions, and crew size.
For a DIY project, expect it to take significantly longer, possibly a week or more, especially if you’re learning as you go.
Related: How much does Home Depot charge to install new windows?