You can quickly set Reps, Timers or Tempos for multiple Playlist items at once using the Playlist Time row (found at the bottom of the Playlist).

There's also a Playlist Time button (looks like a clock) that shows up in a pop-up panel when multiple items, or a Section Header, are selected. Tapping it will bring up the same panel.

If a regular Playlist item is selected, the Playlist Time row will give options for setting Reps/Timer/Tempos for the entire Playlist.