Season matters for price
Window companies are busiest in late spring and summer. Booking in November through February often means a shorter wait, a more experienced crew, and discounting in the 5–15% range.
Installation is fine in cold weather — crews work one opening at a time, and modern sealants cure well below freezing.
Signs it's actually time
Condensation between panes means the seal has failed and the insulating value is gone. Drafts you can feel, sashes that won't stay up, and visible rot in wood frames are the other clear triggers.
Single-pane windows in a cold or hot climate are usually worth replacing on energy grounds alone; going from double to triple pane rarely pays for itself.