The "short" campaign, just to found a tribe, can be squeezed in single evening, but only if you know what you are doing and going for that. I haven't clocked my recent games, but I would advise to plan at least 6 hours for what you could considering successful experience, and that's after you are familiar with the game already. There lot of things you can only learn by trial and error, and in environment where there is no strict right answer for most of your problems that's going to take time.
But the game's very character is more a simulation of being than race to goalposts, where process is more important than achievements. And, you will spend unpredictable amount of time reading unfamiliar mythology until you start thinking like Orlanthi, or the game might be extremely frustrating. Indeed, text comprehension is a critical skill to play this, at least few first times, even despite the wild random number generator that's your main foe and ally (only the final phase of kingdom's building is strictly scripted, most other things can go good or bad way on a whim even when you choose the "right" or "wrong" answer). Later, when you know myths and quirks to heart, you can move forward quite fast, consuming years in minutes, still the events you're waiting for may just not come (even if some are bound to, eventually). Other times you may sit and ponder for a long time, what might be best to do right now, how to do all you need within limited number of turns (there almost always more to do than turns in a year) or how to get out of some seemingly hopeless situation, or just with treasure to use in battle or trade away.
Or even just admire artwork for a while, there events rare enough you may get a new picture you haven't yet seen even after many play throughts, or just notice some intricate details much later. And so on. This isn't a game you ran try peeking in the guide with one eye, it just doesn't make much sense that way, and to try to predict time of an average play is probably futile endeavour as well.
