There hasn't been a ton of new music that I've been obsessed with recently, so I've gone back and started assigning ratings to all the songs that I don't have ratings for in my music library. The last time I did this, I had worked my way up to the early 80s, and after a few more days, I'm now approaching the mid-80s, which is where my real musical taste started to develop as a teenager.
Things should theoretically go faster after this point, because anything I'm encountering now is something that I should be pretty familiar with even if I never bothered to go back and formally rate the songs when I converted everything to digital files 15 years or so ago.
A couple of things I've discovered going through this process: first, if there are unrated songs on an album that otherwise has rated songs on it, then it's highly likely that I'm going to end up giving the unrated songs a 3 (out of 5), which is my rating for "fine, fits the style of the band, but nothing remarkable."
And second, it's remarkable how consistent my taste has stayed over the past few decades: there have been very few cases where I boosted something to a higher rating or dropped it to a lower rating, even though I'd like to think that my tastes have evolved somewhat over all those years, at least in terms of the new music I listen to and love.