How long should pearl barley be boiled for?
It's hard to give a universal answer to questions like this. But, two reputable sources, The Kitchn and Serious Eats each have the same recommendation of 30 minutes.

The article from The Kitchn discussed Hulled Barley (has the bran left on) taking longer to cook than Pearl Barley, which may be why the packages you are seeing have different times.

I see two other possibilities for differing cooking times:


Target market of that brand. For example, a more upscale brand targeting a more "refined" palette may recommend lower cooking times, as these folks will be more accepting of the tender chewiness of properly cooked barley. On the other hand, cheaper brands are likely targeting lay-folk, and let's face it, people without culinary experience might prefer a more done barley.
There could just be some slight variations in types of barley, where it's grown, etc that impact cooking time. This one is difficult to rule out, and leads me to recommending experimentation! Cook a few different batches to different levels of doneness, and see which you prefer.