Quick should be less than 2 minutes, or less than 15 minutes
There are 2 situations that I am often in, depending on this the definition of quick is set. If in doubt, go for the shorter one. Also check out my recommendation to mitigate the problem all together.
Situation 1. Infrequent contact
You have comparatively infrequent contact, and especially almost never ask trivial questions. If the other guy sees you coming, and, he knows that either you have something that is going to take a while or it is not too bad, then quick becomes 'it does not really mess up my daily schedule'. Hence an ESTIMATED time of 15 minutes (if it starts to grow as you go and projected time is raised above 30 mins, evaluate whether you want to continue or plan something instead).
Situation 2. Frequent contact/ Other
If you have more frequent contact, your question distribution is likely not centered around questions that take more than an hour. In that case, anything over 2 minutes can significantly disrupt the task that they are currently doing and hence 2 minutes is quick. (If you have too many 'little things' that is a different problem on which I will not elaborate here.)
If you do not have a working history together or the circumstances are different than usual, just stay safe and assume that quick means less than 2 minutes ESTIMATED time.
Resolution
If your question is quick, simply respond with something like this:
This should be quite quick, {one liner containing key question and
context}
If your question is not quick, respond in line with:
It should be about 20 mins/I don't know/ No, but I need to get it out of the way before T, I want to ask about {one liner containing
key question and context}
Recommendation: Preempt the question whether it is quick
This process is mostly inefficient for short questions (as a significant percentage of time that it takes to answer the question, gets spent on introduction instead). Therefore I trained myself to get in the habit of phrasing things as such:
Can I ask you a quick question? (For quick question)
Can I bother you for 20 minutes? (For questions with decent estimate)
I am stuck with a deadline, could you help me before lunch? (For urgent questions)
I have a question about X (If I don't know too much about the required time, or even whether this is the right person!)
In the last case they may of course still ask you whether it is a quick question. In that case you can reply with:
I am not sure, I want to know {X more detailed but still in 1 sentence}.