Arendiii
Many years ago there was a distinction between:
"Cleared to xxx" and "climb/descend xxx".....one was a clearance meaning at your discretion, and the other was an executive instruction. Over time the distinction has become blurred and in common usage the two are almost interchangeable in modern ATC.
This has led to the practice of using the phrase "climb/descend NOW xxx". Although not strictly correct, it does convey the meaning in an unambiguous manner.......if "now" is omitted, it is not unusual for a pilot to ask..."Is that at my discretion?".......so much so that controllers may actually say "at your discretion".
Speaking personally I have no problem with any pilot who seeks clarification, in fact I commend him/her for doing so. I may also use a tone on occassion which imparts the necessary urgency if I'm busy or the situation demands it.
Remember we're here to provide a service to you without being unnecessarily restrictive.
I once did a familiarisation flight and the pilot said that when given a descent clearance from cruise he would retard the throttles very gradually to prevent rapid heat/temp change in the engines.....no problem at all for me....but if you haven't begun the level change within a minute [long time in ATC] expect a reminder or a repeat.
You could be expected to maintain current level for longer if you were given an instruction such as "level by xxx".....because it would be implicit in the clearance that descent [or more rarely climb] would be calculated by the pilot to suit the operating characteristics of the particular aircraft type.