Hey there, this is a great question!
A Premiere Pro timeline can be as long or as short as you want it to be, but there's no specific setting to lock it in at a certain length. You can set "in" and "out" points to identify a work area anywhere in the timeline (doesn't have to start at 0:00) by pressing I on your keyboard for an "in" or start point, and O on your keyboard for an "out" or end point. I do this when working on promos that need to be a specific length! When you export your video, it respects those in and out points as your sequence and ignores the rest, so keep a close eye on them. After setting them, you can clear them by right-clicking within the space and select "clear in and out points."

I would use the in and out points to identify the sequence length you want. More timeline tips & tricks can be found on our Youtube channel, I particularly like the "tips to edit fast" with Valentina Vee.
Hope this helps!
Caroline