How far are shortened replies to ATC allowed to go? Pilots often use non-standard-phraseology for readbacks. Some examples:


  ATC: DLM, switch to tower, 121.2
  
  P: tower, for DLM


or


  ATC: DLM, identified, climb via the RIDA9 departure
  
  P: roger, climb via, DLM


or


  DLM, cleared for takeoff runway 17
  
  P: cleared for takeoff, DLM


The last two example are from two videos by Boldmethod, How To Fly An IFR Departure Procedure With A "Climb Via" and How To Fly An Instrument Arrival Procedure (STAR), used by an ExpressJet-pilot.

Especially the two last ones were a bit shocking to me when I heard a professional pilot using them. So I now want to ask, how far is this "ATC-Slang" allowed to go?
ATC is required to always use standard phraseology, and tapes are randomly reviewed to verify this--and too many violations gets them pulled off for retraining. This makes sense since talking is literally their entire job and it could be with anyone from students to veterans, including people who don't natively speak the language.

Pilots can use whatever wording they want as long as ATC can understand what they're saying, which naturally leads to shortcuts.  It saves time/effort and helps keep the frequency clear for others. In fact, if/how you abbreviate things is a sign to ATC of how experienced you are. If you read back "climb and maintain three thousand five hundred feet", they'll treat you a bit differently than if you say "up to three point five". Listen to a busy airport or approach on LiveATC and you'll pick up the patterns in just a few minutes.

"Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away." --Antoine de Saint-Exupery