The game should explain to us exactly how long that would be
Where's the fun of that? I much prefer variables that change over a range only approximately known, as a result of the interactions of many variables, the weights and even identities of which are only approximately known. This makes the game a complex puzzle, which is what I like

.
That is an interesting goal point. So your saying that a guests needs are "usually/approximately" 2 drinks 1 food and maybe a souvenir for them to leave happy? I know each demographic is slightly different but, the point is guests never stay in a park long enough to eat 5 meals, right?? They still seem to stick around for a very long time, especially if you want them to reach the back of large parks
It all depends. Like I said, it's a complex puzzle for which only some general observations can be made. And because all parks are different, and the same park varies over time, those observations won't apply the same way at all times and all places. At most, you can just point out some trends that are important to observe, and some of the things to do when such trends happen.
A peep's decision to leave the park works on a mechanism independent of the park's overall pricing strategy. After all, peeps don't know and don't remember prices. According to research by @kickflip, which my own observations seem to confirm, leaving the park is always an option whenever a peep is deciding what to do next. The attractiveness of leaving the park starts out very low but increases over time. Anything that causes unhappiness speeds up the growth of the attractiveness of leaving, anything that causes or maintains high happiness slows it down. But the growth never stops, so nobody stays in the park forever.
So, at any given point in a park's development, peeps of the various demographics will be staying in the park some average amount of time. You can see this on the Guests tab of the park management display. This is just an average, though, the center of a wide distribution. You can also see on average how much each demographic spends on food, drink, and gifts during their average time in the park. If you know the prices of your merchandise, you can thus estimate how many items of each type the peep will probably need to buy to sustain him for that amount of time, but you also have to factor in the frequency of peeps complaining of being hungry or thirsty, or turned away because the shop queue was full, or they couldn't afford to buy anything. Anyway, taking these numbers into account, then giving some wiggle room on top to account for more than half of the population (the average being only the midpoint), should give you some valuable info on how to price your stuff, regardless of which strategy you use.
Just keep in mind, however, that the bigger the park, and the more stuff it has in it that caters to the tastes of each demographic, the longer peeps will tend to stay. They have more park to explore, more rides to go on, etc. The longer they stay, the more food and drink they need, and the more likely they are to want to buy a gift. Thus, you'll have to adjust prices, probably downwards, as the park grows. But OTOH, the bigger the park, the more peeps it attracts, so the more money you make in total.