windows - How to use slow motion effect in a specific time interval with ffmpeg

I'm trying to use slow motion effect on my videos. Let's say I have a 2 minutes long video and I want to use this effect between 20 and 30 seconds.

I found this command from a blog:

ffmpeg -i input.mp4 -vf "setpts=(/1)*PTS" output.mp4

But i don't know how to implement duration in it.