Well, you can't keep it at 85F forever and feed it every few hours for the rest of your life...
Here's the deal: The culture is a bunch of organisms that just want to eat and reproduce. That's pretty much all they do. If there's food around, they eat it. Ths speed at which they eat is primarily a function of the temperature. The 2 different kinds of organism in a culture (yeast and lactobacilli) are affected by temperature differently, but regardless, the warmer it is, the faster both eat and reproduce. As is eats it gives off waste products. These are CO2 which makes bubbles, and various organic compounds such as alcohols, lactic acids and other yummy things. Once it's eaten all the food, it's also expelled a lot of waste and starts kind of swimming in it's own waste and gets nasty. It sounds kind of gross, but that's what fermentation is all about. After it has degraded it's environment, it doesn't just die off, but it does slow down alot. The truth is that you can keep a culture out at room temp for a few weeks at least, without feeding it or cleaning it up, and it will still live. But it will just get progressively nastier and nastier looking, with green liquid filling up the top of the container and the flour sinking to the bottom. Actually, if a sample is really old and not fed, it will start to smell like wine. It looks nasty, but I've had people close their eyes and just smell it and guess what it is and they guess wine everytime. No surprise, wine is fermented by yeast for long periods. It really looks dead at that point, but I've never had a culture not revive. I had one in the fridge unfed for 6 months, and it looked nasty as anything and then I fed it a few cycles and it looked bubbly and healthy.
When you are not using a culture, you store it in the fridge. It will slowly degrade and develop hooch (that green liquid). It could take a day, it could take 3 weeks but eventually that is what will happen. Now you want to use it. You have to feed it and bring it back from that dormant state before you put it in your dough. How long does that take? It's really depends. There's no set time. It could look great after 1 feeding and 5 hours, or it might take 5 feedings and 3 days. But eventually, it will happen. You are on it's schedule. As I said before, you know it's time to feed it when it starts to look worse. At hour 0 (you just fed it), it looks like a flat batter. At hour 3 there are some bubbles. At hour 5 there are few more bubbles . At hour 6, the bubbles are deflating and it's looking slimy on top. In other words, it got a little worse. It's peeked. Time to feed it. Now, it peeked but it hasn't peeked well. Hour 5 was it's peek, but still not that great. So you have to cycle again. The next peek should be faster AND better. The second time, that cycle may take 4 hours. Why? Going into round 2, there are more live ones and so the whole thing is going to go faster.
If the culture is really old and slimy, the first peek might not look like much at all. But the same rule applies. It's relative. As long as it looks better right now than it did 10 minutes ago, it can keep going. As soon as it looks worse than it did 10 minutes ago, it can be fed. You don't have to sit there and stare at it, but that's the general idea. If you wait too long to feed it, then it will start to die off and will require more cycles. This happens to me all the time. I keep missing the peeks because I've got other things to do, and it so takes days to get the culture ready. I would say that the 12 hour time is not really a rule, but rather an upper bound. If it's sat for 12 hours and kind of looks the same, with no peek at all, then you can just time it out at 12 hours and feed it then. But really if you get to know it you will see that there's always some discernable difference, and thus always something of a peek. I guess there's one other case. And that is that the peek is hour 0. You feed it and 10 minutes later it already looks worse. So it's like it peeked at the start. In that case, let it go a few hours. Maybe 4. It might pickup a bit. If not then go ahead and feed it again.
As far as temperature goes, I no longer culture it at 85. It will go faster at that temp, but I just don't bother much anymore. I just do it at room temp. It's slower but otherwise the same. The only time you really have to do it at 85 is if the culture is like 6 months old. Otherwise, it should come back in a few feedings.
The good news is that there really is no way to screw this up. It's pretty hard to kill and you can always start over. The only real way to kill it is to contaminate it with a competing sample.