The Bestiary has tables to randomly determine what kind of adventure you see in a given area, but what are the chances per day for a random encounter?
I really think that varies from area to area.
Trying to specify something like "Random encounters should be rolled 4 times per day with a 20% chance of occurrence on each roll. This applies to every region of every continent on every world and on every plane, as well as at sea, undersea, underground, in dungeons, in tombs, in cities, and even in the king's royal treasure room." would be a little awkward.
So the core rules (Rulebook and Bestiary) leave it up to the people who write each adventure to specify things like "The mountains to the north are crawling with monsters, so make 6 encounter checks per day with 40% chance each time, but the grasslands around the capital city are extremely safe, so only 1 encounter check per day with only a 10% chance..." and so on.