Well, what do you think? Besides the main shows we have a few Animated Spin-Offs, as well as comics, but it's clear that those 4 shows are the heart of CW's superhero verse. How much longer do you think they will be around? I'll start.
Arrow: Well, it's into it 6th Season now, and even though Season 5 was a huge improvement over the horrendous Season 3 and 4 (and Season 6, so far, keeps up that standart), the ratings just haven't been that good lately, and with the flashbacks having ended and obviously many plot points at once being resolved during Season 6, I think this might very well be the final Season of the series that started it all. Sad (for me at least), but maybe Oliver and co will keep making cameos in the other shows.
Flash: Well, the Flash (despite Season 3 having been a bit underwhelming) is easily the most popular of the bunch. I don't see it going away for quite a while, just like Smallville back then, I think it will be renewed at least for a 7th Season.
Supergirl: This one is a bit difficult for me to predict, but I think it might end after maybe 5/6 seasons or so?
Legends of Tomorrow: Just as The Flash is definitely the most popular, Legends is, without a doubt, the most obscure of the bunch. And even though I really dig the ensemble and think it has greatly improved starting with Season 2, Time Travel is a concept you can only repeat for so long, especially in a verse where the rules change like every episode. I think it will end after Season 4, at BEST Season 5.