How long can a instagram video be?

You can upload longer videos, but the best length depends on your content. Short and engaging videos often perform better.

I think up to 90 sec on feed and longer for reels now

Depend on the content type mostly 30 sec to 1 minute.

Videos that are around 20-60 seconds long will have better quality.

I usually see videos that are around 30-60 seconds long.

It depends upon type of the content which you are posting on instagram. Usually for common reels the duration is almost 30 to 90 seconds. But if your content is storytelling or explanation then the duration can be increased upto 3 minutes, but if you are posting common reels like memes or other entertaining content then 30 to 90 seconds is best duration. It also depends upon you too that how quick you explain or present your content.