Hi G4RAY,
Thank you for using the Apple Support Communities! We understand you're trying to use dictation within Pages on your iMac, but it seems to be cutting you short around 15-30 second mark. We'd like to see how we can help.
Based on what we found here Use dictation to enter text in Pages on Mac - Apple Support it does note, as quoted, "Note: For best results, speak for no more than 40 seconds at a time." Which tells us you shouldn't be cut off before then at least.
Are you following those specific steps in the article to use dictation? Are you seeing that cut off at 15-30 seconds if you dictate within another application? Which macOS are you using under the Apple menu > About This Mac?
Best regards.