I can’t really comment on exactly when VXGI (which to me seems like the most promising realtime GI-solution available atm) will be viable to use, but I think it’ll be 1-2 more graphics cards generations until you can use it freely in big scenes, since it’s quite a resource hog. My (admittely very limited) tests show that my framerate (Titan X with a i7-5720 CPU and 48Gb memory) drops by about 40 fps when enabling VXGI. My tests were done on UE4.10, and some things might have changed since then.
I would think that VXGI would run smoothly on the next generation of Titan, but that’s a very expensive card, so you’d probably have to wait one more generation in order to use it on midrange hardware.