TL;DR: What percent chance should my players have of running into a fight that's not directly connected to the storyline?
I've set it up so that they're using a spell to lead them from destination to destination in a huge rainforest. The forest is experiencing a weird overgrowth problem that is making all the animals Dire and violent. I want the spell to have a "fail percentage" so that they occasionally run into fights where they can see how this overgrowth is affecting the creatures.
They'll be using the spell once or twice a session, and sessions are about three hours long. I'm leveling them up VERY rapidly (for reasons that are probably obvious) - a level every session or two. So, the fights aren't necessary for leveling EXP. In addition, battles with this group tend to gobble up a lot of play time for some reason that we're still working on.
I tend heavily to the side of the spectrum that says "roleplay and build character story first, fight where needed", so I was hoping for some less-biased opinions on how often these "random" (but not really random) battles should happen. I was thinking a 20-25% chance?