How long does it generally take to release a soundtrack? Essentially, I want to know how long it normally takes for soundtracks to be released after an anime's release. Are the soundtracks always released? 

The main reason I ask is because I want to know when we can expect 
K: Return of Kings' soundtrack to release. It came out in 2015, and I still haven't heard any news about its soundtrack.
This doesn't answer your broader question, but the K: Return of Kings soundtrack is scheduled for release on April 27, 2016.


  Are the soundtracks always released?


Not always, but I'd guesstimate that 80-90% of anime have soundtracks released at some point. Those that don't are mostly shows targeted at children, who are probably not likely to buy soundtracks.


  Essentially, I want to know how long it normally takes for soundtracks to be released after an anime's release.


This can range anywhere from "before the show is even done airing" (e.g. Shokugeki no Soma's first soundtrack album) to "not until season 2" (e.g. Nisekoi's first soundtrack album) to "yeah, we were going to release it soon, but lolno too bad" (e.g. Tokyo Ghoul - granted, they did put out an actual album half a year later).