I have a classical exam next week and need to play 30 minutes.
With the pieces i'm playing I only have 23 minutes or so. I don't have time to restudy a villa lobos etude or so that will be good enough for an exam.
I never needed to play so long so would it be a big deal that i didn't play exactly 30 minutes?
It's an exam to enter in a Conservatory
^ lol
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^made me lol aswell.

But can somebody who has done such an exam answer to the question?
In my experience 23 minutes will be fine for an exam unless they are anal retentive about it.

Can you ask if 30 minutes is a firm requirement? Usually it isn't. Better to know ahead of time though.
In general, if exams of this sort specify 30 minutes, you should try hard to come reasonably close to 30 minutes. But unless it's spelled out that, say, anything under 27 minutes will disqualify you, or that you will be thrown out of the room at the 31 minute mark even if you're still playing, you probably needn't take the 30 minute rule as having zero flexibility.

And in general, 23 solidly excellent minutes is far superior to 30 uneven minutes.

But if at all possible, try to find out how strictly the 30 minute thing is taken. I'd do this by contacting the admissions people at the conservatory (or equivalent) and asking. If you're told it's just a general guideline, you can go in without worrying about this point. If you're told otherwise, at least you'll know and you'll be able to prepare yourself as best you can.

Otherwise, you may play beautifully for 23 minutes, only to have one of your examiners say, "That was very nice. But I see you have about 7 minutes left. Please play something else now.

You have until next week. If necessary, that's enough study time to learn a piece well enough, if you push yourself hard. And pushing yourself, and working under deadline pressure, are qualifies that will stand you in good stead during your years at the conservatory and beyond.