How cold is too cold to wash windows?

There is no single cutoff temperature, which is why the usual "above freezing" answer is incomplete. Professionals work below 40°F routinely by adjusting the solution and using less water. The real limits are the temperature difference across the glass, whether the frame can drain before it freezes, and whether the pane will dry. A sunny, breezy 38°F day beats an overcast, damp 48°F one.

Can hot water really crack a cold window?

Yes, and the numbers are not close. Viracon's thermal stress bulletin notes that each 1°F of temperature difference across a pane creates about 50 psi of tensile stress, while a clean-cut glass edge holds only around 2,400 psi - so roughly a 50°F differential is enough to break annealed glass. Hot water on a frozen window clears that gap instantly. Use cool or lukewarm water instead.

Is it better to wait until spring?

Sometimes, and we will say so. Wait if your windows were done in late summer and no trees overhang the house, if a hard freeze is within 48 hours, or if the haze is sealed between the panes. Do it now if the exterior glass is visibly dirty, if gutters have been sheeting water across it, or if you want clear views through the darkest months of the year.

Should I get inside and outside, or outside only, this late?

Outside only is usually the better late-season buy. The exterior is the half you cannot safely reach in cold weather, and it is $3 per window versus $6 for inside and outside. Interiors are warm, safe, and doable any weekend of the winter. Add window frames and tracks at $5 each so drainage is clear before the first freeze.