TL;DR: How early can windows made of transparent glass (can look through) be readily available (not a rare luxury) to "common" citizens (at least top 10% of population)?
Disclaimer: I know very little about the history of glassmaking or the modern processes involved. I dabbled a bit looking for an answer, and keep doing so, but maybe someone is an expert and can provide a better answer.
Glass was known to humans for quite some time, and as early as Romans (and probably earlier) it was used for windows. However, for a long time produced glass had poor optical qualities, so while it would let the light pass, it wouldn't allow you to see through a window. There is some evidence pointing towards transparent glass in the Ancient Rome and Greece, including references to magnifying glasses, but nothing consistent. After the Dark Ages, it took some time to recover glassmaking knowledge, and even by the 18th century glass windows were a luxury even for the highest society.
As with many things, what was a luxury, now costs less than the same amount of bread or vegetables. The question is, is it purely our technological superiority that allows for this? Or are there some glassmaking methods that would allow, if one were to know how, to make transparent glass relatively easily and cheaply? If this is merely a matter of knowledge and invention, then how far back can we go and still have good enough technologies to make glass for the masses? Can we have it in early Industrial age? In the Middle ages? Back in Roman times?