If you guys want to wait for big price drops it will likely be more than two years. Mainly because the big price drops usually come after shrinking the lithography. The whole time the PS4 was on 28nm lithography it was a $400 system. As soon as manufacturing went to 16nm in 2016 they discontinued the PS4 and replaced with the PS4 Slim and PS4 Pro, and then you started seeing $230 or so PS4 Slim pretty regularly. But chip makers had been manufacturing on 28nm since 2011 or so.
The PS5 will be on 7nm and that's still a really new process. So it might be quite a wait until we get a node shrink from 7nm and a PS5 Slim. Especially since node shrinks are taking longer now. In last gen PS3 launched at 90nm, got shrunk to 65nm in the last gen of PS3 fat, and went to 45nm with the Slim (I think the Super Slim stayed on 45nm).
I'll probably wait for a reasonable exclusive game library to get built up unless either Ghost of Tsushima and Death Stranding are truly amazing and justify the upgrade or if there are killer PS4 trade-in deals to the PS5. I think the former is probably more likely since I guess PS4 will have complete crap value once PS5 launches thanks to the BC.