I agree with KneesUp et al that duration of ride in itself in isolation to other factors (particularly effort) does not mean much.
Also, imho, the original question was open to different interpretations because both duration of time and distance are commonly quantified as short or long, and in the context of bike rides, distance is determined by duration and speed. I could answer the question "Is a cycle ride of 30 mins a short one?" with "It depends how fast I'm going - the slower my speed, the shorter my ride will be". Though I would also say that giving a little thought in answering the question and it would be difficult to interpret it other than how the OP intended.
When I ride without any purpose other than riding (by that I mean not for transport and not for utility rides), I usually do a loop from my house that I can vary in length substantially. My shortest loop takes me about 20~25 mins. My next loop up takes about 40~45 mins. Most of my rides are between 1 hour and 2½ hours so in that respect, 30mins would be a short duration ride for me. It would also be a ride of short distance - I average about 10~12 mph, I almost always ride alone, I don't ride for sport and I never use Strava or chase PBs.