But points have been made about the abundance of replay value, so I am saying around 3-4 totally and utterly unique playthroughs may be possible, so let's say the game has
700 hours worth of content?
The question of the number of utterly unique playthroughs (and how long they might take) tickles my math brain; I'd like to see if I can formulate something. How utterly unique do you need them to be, though? Are we talking : unique dialogue all the way through? unique quests from one run to another? unique decisions and outcomes of quests, ie doing the same quest in several runs as long as those quests play out differently?
So I'll give you my logic:
An utterly unique playthrough needs to be unique in its story, dialogue, gameplay, and build variety. As it stands, there are 10 companions, and 2 custom character origins, for a total of 12. These companions and origins tend to evoke different responses from the story based on the situation you are in and who you have. Playing an origin also gives a unique flavor. That means you can have 1 party with Tav and 3 companions completely different from a Dark Urge with another 3, completely different from an Origin-Companion run with another 3. Further, the majority of these characters have different classes. Further, two of the three unrepresented classes of Sorcerer, Monk, and Bard can be filled in by you (or you can do 4 runs with only Tav or Dark Urge PC and fill one last run with a hireling and do all unrepresented classes). Also potential for a Lone Wolf Run (probably best as Dark Urge???)
Next are the moral decisions you make. There is clearly a "good" run and an "evil" run. You can probably also get a different run by doing a mixed neutral run, and perhaps by doing a combat-minimal run with stealth and dialogue.
Order of events seems to affect how things occur.