In 20+ years of working on my house, I've only had to replace glass twice at
times when the sash had to be reinstalled quickly because of bad weather. In
both cases, I primed & painted the sashes within 24 hours of applying the
putty, which was still soft, but had formed a skin. And, in both cases, I
completely forgot to check on the situation until months later, and found
that the putty had hardened properly. In other words, I have no idea if the
putty hardened in days, weeks, or months.
I just installed new glass in another sash, but this one could sit on the
bench in the basement for a few days, if necessary. Should this be enough
for hardening? If not, how long is usually about right?