Okay, that's a different concept of read-time than we've talked about on this site in the past. The references you've probably found are related to the date and time that a user last read an article (last read time). This is stored in a field called readtime for logged in users.
Thank you for clarifying that you want a measure of how long it might take to read an article. For non-wiki content where templates are not used, this math is simple enough to estimate. But since templates might be used within wiki content, time-to-read would have to be calculated when an article is saved, because it needs to be fully parsed out to do the calculation.