Saying "X time is too long" is pretty silly stuff and means you aren't understanding your role in this, as a DM. If you set up a really complicated situation, and make it seem really high stakes, and the players react by planning elaborately, you are to blame, not the players, and putting a timer on them, rather than apologising and asking if they could do it within a certain timeframe is dubious DMing, I would suggest.
Pacing is always the GM's responsibility, and probably one of the hardest things to learn. It can be especially hard for some GMs if they don't feel they should have any influence over what the players do in regards to roleplaying and let the group argue for hours or let that
one player spend an hour buying rope.
It's okay to cut them short if they are spinning their wheels. It's okay to interject with clarifying information or suggestions based on what the characters reasonably would know or have experienced. It's okay to tell the players they have already spent too much time on this and they have 5 more minutes.
GMs must be able to read the table. It's a required skill for avoiding people hogging the spotlight or becoming bored. And reading the table means figuring out why they are spinning their wheels and helping them out of the rut.
Move the game along. Openly endorsing plan A, B or C. Or force their hand by outside influence. Or have bad guys break down the door.
The OP isn't terribly specific, but my guess is ambivalence on the part of the consulted NPC to Plan Acontributed to the over planning, and the player that suggested Plan.C just didn't like Plan B. If the GM was trying to warn the PCs off a certain course of action with the NPc, that's fine but probably suggest an alternate. If the GM was just "playing the NPC in character" and caused everything to come to.a screeching halt, don't do that.
To reiterate, pacing is the GM's job and one of their most important ones. In and out of combat. Rein players in when they are spinning their wheels, as a group or individually, and don't contribute to the slowdown by having NPCs disrupt the players' decisions. In the worst case scenario, force them to make a choice, with guidance if it seems reasonable.