If you try to grow mold or fungus (think: mushrooms), you will find that it generally takes at least 7 days under near-ideal conditions for a spore to reproduce to the point where it's visible. This is why most food will be edible in a refrigerator for at least a week (not necessarily fresh or tasty, but at least visibly mold-free). It's also why most food with at least some moisture will show mold after 14 days or so (mold spores are pretty much always in the air, so the moment you expose food to it, the countdown begins).
Ideal conditions for sporulating microbes include a stable temperature range, aqueous environment with a particular pH, consistent food source, and for some microbes, oxygen. Thus, non-ideal conditions include desiccation, food scarcity, and movement. The parts of your hydration system that get regularly cycled with water and have little exposure to air (like the bladder) will be relatively inhospitable to microbes, due to the poor food supply (assuming you get mostly clean water) and low oxygen. I've never seen biofilm on the inside of my water bladders.
On the other hand, the parts which are exposed to oxygen and get a regular "food supply" (like the mouthpiece) are much more likely to accumulate biofilm. Fortunately, people drink out of water bottles with visible biofilm every day, with little to no ill effects. But in my experience, it noticeably changes the taste of water. So if you can taste the biofilm in your system, it's a good time to clean it out (which means, not a bad idea to clean it out before you go on your trip, if you haven't for a while). If you can't taste the biofilm, then you're gonna live. Your Stone Age ancestors were exposed to much more mycotoxin than that, and yet, here you are!
If you're really paranoid, just wiping off your mouthpiece in clean water periodically will keep the biofilm to below-detectable levels. But you'd need to be pretty paranoid to do this more than once a week. The tiny internal bits in the hoses are more difficult to clean, but unless you're backwashing bits of food into your water system, it shouldn't build up much.