While planning my trip, I was looking at some trails. They all say x amount of hours, but I didn't know if that was to the peak or if that was back and forth. What is the general rule?
I'm not sure I've seen an x hour hike advertised that hasn't been a round trip (unless explicitly stated otherwise.) This makes sense since a round trip is what almost everyone will be doing, and if it was the other way around and someone didn't realise, potentially dangerous situations could ensue!
EDIT: As Vorac points out, this clearly isn't always the case as I first thought - so it pays to check!
In my experience, mostly in North America, an X hour hike provided by a park service is typically given for the round trip unless stated otherwise. You can also confirm by looking at the mileage and elevation.
For example, Red Rock provides round-trip hour estimates. For example, Turtlehead Peak takes around 3-5 hours for a round trip; definitely not 6 to 10 hours (at least for an average hiker).