How far are shortened replies to ATC 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