If it's just an instrumental (assuming an urban type genre), maybe an hour and a half to two hours. But this is largely because if it's just an instrumental, the level of detail isn't as high as it would be for a normal record. Instrumentals are mixed because they need to be shopped to artists. Because there's no vocal and because it's not a finished record, there's a lot more leeway in terms of what you do with the mix. If it's an EDM instrumental, or jazz (or just a jazz soloist over an urban bed of music), or rock instrumental, etc., it's going to take longer.
For an actual song, assuming major label type quality (whatever that means LOL), it's a day to a day and a half usually (8 - 12 hours). It's a lot longer for a few reasons. First, you have the vocals. Second, when you add vocals, the music becomes a little more difficult because it's now interacting with something. Third, it's an actual record for salable release so the nit-pickiness on quality goes up quite a lot (even if it's not being released commercially, I still treat it that way). Fourth, you have to run numerous passes (main, instrumental, a cappella, tv track, and possibly a clean version, minimum) and that takes time.