Well, when I said "just apps" I meant "apps and their associated data, everything except pictures, music, podcasts, etc." (which I keep on the SD). So on my current 16G system I lose upward of 6G to the OS. Then there's about 6.5G "used", 1.5G "cached" and a little under a gig "misc". That leaves about a gig free, and I just deleted 512M worth of samsung text to speech bloatware yesterday. So it's not completely full, but it's at the point where I'll occasionally have a large application update fail because of low disk space--and I'd rather just not ever deal with that. And all that is without much in the way of games or movies or other really big space hogs. It's possible to get by with less space but at some point I guess you'd kinda just be better off with a dumbphone if you don't want to do anything on the smartphone.
My wife's 32G 5x has something like 26G available after the OS, so I'd assume a 16G 5x would also have about 10G after the OS. More than enough for her, but the kids these days burn through storage like I can't even fathom.