Show total ad time left instead of or in addition to the number of ads left.
When a streamer runs a block of ads, we select the amount of time we want the ads to run for, and that can be anywhere from 30 seconds to 3 minutes. However when we do that, all the viewer sees is the number of ads left, not the total time left. Since ads vary in length, that doesn't actually tell them how long until they're over.
Speaking for myself, it's pretty much a daily occurance to have someone post a message like "8 ads??" in my chat. And sometimes that number can be in the double digits. If viewers were either instead or in addition shown the total time left, like we select as the streamer, I think it'd reduce the shock of how many ads are coming, which should make for a better viewer experience.
Also kind of a side point, but if ad inventory is a problem, you could just display "Up to x:xx of ads left." I don't think anyone is going to complain about the timer being cut short.

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Yagamoth commented
I fully agree with this sentiment, thank you for bringing this up