You can adjust the length of the entire timeline itself for all of your banners by clicking on the Timeline select box in the Bannerify header; selecting a length (with options from 3 seconds to 3 minutes) will automatically extend or shrink the Bannerify timeline for all of your banners. Bannerify will do its best to update your existing animations if they previously extended beyond a new shorter timeline, and vice-versa.

You can't automatically undo timeline length updates. Selecting a new timeline length will apply the changes to the timelines across all of your banners. You will need to manually update your timeline animations again if you change your mind. With this in mind, it's usually worth setting your timeline length before starting to work on your animations.