I can’t think of any way to do that automatically with Audacity, but if you tell us what the job it (why do you want to add 3 seconds silence every 25 seconds in a 10 hour recording?) then we may be able to suggest an alternative.
I’m using Audacity to “read”, and the embedded dictation from the mac. It works like a charm, except that the dictation tool has a 30 s “memory” so to speak so it can write what it ‘heard’. Hence why I want to audio to pause every 25 secs, this should increase the accuracy and ensure I’m not missing big chunks of the audio transcription.
And I can’t afford professional transcript of almost 10h of audio…
I’ve not use the Mac dictation tool, but is the basic problem that the tool cannot keep up with the text?
I don’t think that inserting regularly spaced silences will work very well because it will cause some words to be split.
The first think that I would do is to split the 10 hour recording into more manageable size sections of say between 30 mins and 1 hour.
Then there is a plug-in available that can extend silences, which can be used to increase the spaces between words and sentences: https://forum.audacityteam.org/t/extend-silences/34612/1
If you attempt to use the plug-in on the full 10 hours I would expect it to crash due to running out of available ram. Test it on a short selection first, and if it works, try increasing the length of the selection, but I’d recommend no more than an hour max even if your Mac has loads of ram (Nyquist is strictly 32-bit and can’t access more than 2 GB of ram).