It depends on the mode. They can choose the time, but then get awful poor ad-revenue. Or they take the juicy contract, and are forced to let twitch decide when ads are rolling. And with gaming content, even choosing the time is not always flowing well with your content.
All of you talking about how streamers just can't pick when to run an ad have no idea what you're talking about.
They can schedule a few 15 second ads and then run longer ads manually to fulfill their 50-50 revenue share / preroll exemption.