The 5min example is like a maximum/extreme case, yes. My average time spent writing each prompt is probably 30 seconds or less, and coding time saved per prompt like 25 to 60 minutes.

When I do spend minutes (not seconds) writing prompts, it's because I'm actually typing a "Context File" which describes with full clarity certain aspects of my architecture that are relevant to an Agent task set. This context file might have constraints and rules I want the Agent to follow; so I type it once and reference it from like 10 to 20 prompts perhaps. I also keep the prompt files as an archive for the future, so I can always go back and see what my original thoughts were. Also the context files help me do system documentation later.