When you need to present a new idea of a software to stakeholders, you let them decide the duration of the presentation or you decide? I'm facing a very fucked public saturday but it's an important project

I'm not an expert in this, but I would set it this way:

Duration Anounced: 30min
Duration Planed: 15min

You have some spare time if someone is late and managers are ALWAYS late... Plus some time for discussions afterwards.
Also prepare some more "slides" with expected questions (like how do I do this? -> oh just click here, cool animation, stakeholder happy)

Plus keep in mind, no matter how long your presentation is, people won't take away more than 3 things.

Another "tactics" is to drop a bomb at the beginning: If we don't do something, we'll lose X thousend $ in the future, we've got solutions A, B and C, we reccommend you A because of X, Y, Z

@Wack Great answer! Thanks