This assumes that you will have time to get them *all*, Jimena in the streams has multiple kings of their dynasty netting them a cool 4.8 renown per month (most of which is from independent rulers, only 0.8 is from total dynasty members). This results in about 17 years 5 months for an unlock of 1000. Unlocking *just* all of the initial legacies would take a total of 122 years. To unlock every legacy in a path, would be 261 years at this rate, that's over 1800 years for all of them. Therefore, you are probably looking at one or 2 completed paths with some dips into others.
This, of course, is related to the Jimena shown, but this is actually one of the larger values thus far. We also don't know if we will get renown from events and how much (realistically) a dynasty head will spend on their shenanigans. Still, if you want to see those powerful bonuses during a playthrough, you will probably need to invest heavily in *that particular path*. Now, it is certainly possible that players set up independent kings and emperors of their dynasty all over the map to try and boost their renown, and there are some special buildings and such that give minor bonuses, but for most situations I don't think we are looking at more than a couple completed paths, which makes sense given that the devs have said they want these to make dynasties more unique and interesting.