This is one of those diffuclt lines. In general, players tell you what their character does. If they give in and succumb to torture, they're losing the ability to tell you what they do - similarl to how they'd lose control over their character when they are charmed or dominated.
I'm not a fan of having to roleplay torture until the player breaks

..but I did read that book...
This gets a bit on the topic of social mechanics:
Along these same lines of thinking, I prefer a solid mechanic for social consequences of "serious" things (e.g. insanity, stress, etc.), rather than a hand-waiving by the players. The reason I prefer that is that I don't believe that a player should be off the hook when they make certain stats "dump stats." There should be consequences to having a low charisma or low wisdom (willpower). We have a mechanic for combat that cannot be hand-waived off by a player, so I prefer games (and game systems) where significant social mechanics have similar consequences. Players who put their points into cha/wis should be mechanically a LOT better inside that system, just as a fighter is good at his combat-monkey stuff.
Some people hate social mechanics (more power to you). My two groups tend to play games with social mechanics by choice (Warhammer Fantasy, for example). I think D&D5e can handle it just as well with just a touch of thought put into that the previous iterations of "4 failed saves and you convince the king to sleep with the half-orc." it should lean towards this: a mechanic that offers players directions of choice. Ok, you failed 5 times to convince the crowd not to burn the witch and you have a twitchy finger on the crossbow...do YOU believe your character would hold out that long?
It is justified by the dice roll, not the finally determined by it.
I'm not talking about being seduced by a troll into an uncomfortable situation b/c you fail a single (or 6) wisdom saves, but instead, ongoing, consequential effects for characters who perhaps want to play something better than just combat..and this without spending 14 hours of game play romancing the DM...
Any thoughts how long to let a character try to hold out on the torture (physical and/or insanity) thing?
jh