I can hold and drag each individual layer on timeline to increase its duration but it's such a pain.
Is there a workaround to change (increase) all layers on timeline to have the same duration?
This is a bit late, but...
You know how you can select multiple files by
You can do something similar here.
This will select all the desired frames. All you have to do is then change the duration of one and it will apply to all selected.
For this to work, gotta be in "frame animation" mode vs. "timeline animation" mode
You might be interested in The Animator’s Toolbar:
@PS_Animation tweeted: Ever tried trimming multiple layers at once, with a single click?
A new update (2.5) is also coming soon, which supports moving and trimming multiple layers, even inside Video Groups.
If I understand your question correctly, changing the fps setting will change all layers' duration proportional to the length of the clip. I was working on an animation that was intended to be part of a set of small multiples. I wanted different instances to move more quickly than others, so rather than getting all tweaky I changed the frames per second on the ones I wanted to go really fast, and the ones I wanted to go really slow, and exported them. It worked for my purposes, but again, I'm not sure that's what you're asking.