Controle technique

Depends what it fails on.
You have 15 days to fix anything minor or non-critical
Your can can be banned from public roads completely of it's dangerous.

If a vehicle fails the test, a certificate is issued, valid for three months. The necessary repairs must be carried out and the vehicle returned to the original test centre for a re-test within this period.

If there are grave concerns about the vehicle's safety it may be issued with a red certificate, which is valid for 15 days or, in extreme cases, it may state that the vehicle should not be driven until the problems have been rectified.

I cannot recall when my car last failed the CT but I'm sure that I didn't have 3 months to get it repaired.
Two things to be aware of:
You can also get 'keep an eye on this' on a pass certificate for things that don't need immediate attention but which are likely to need dealing with before the next test.
When you return for re-test, there is a special line and you will only be re-tested on the things on which the car was failed - shorter wait and much quicker overall.

The 3 month perid is a rather rare exception. The rule is a red certificate that allows you to travel only to a garage and to the control center again, and it is valid for two weeks. You CANNOT use it to drive around. If you return for the second check within the 2 weeks, you only have the part in question checked and you pay a small amount. If you return after the 2 weeks, the car goes through a complete check again, and you pay the full amount.

Ive only ever received a red one on fails - 10 days to get fixed and return.

I've never seen the logic of getting a garage to do a pre-test.
They may not find everything that the CT will fail on. They certainly may find - and repair and charge you for - things that the CT will not fail on.
If you fail the CT you then know exactly what needs repairing and you pay €12.00 for the re-test.

I'm inclined to agree with kasseistamper on this one. If you fail, they tell you what needs to be done, and you can go to a garage and ask for that specific repair or adjustment to be made.

OK if you trust your garagiste, or if it's timed close to your regular checkup / service, then fine.

But otherwise you're essentially inviting the garage to look for problems, and then to bill you for fixing them, and you aren't even sure that it is something that may have failed your car.

Yes - Pre-tests are a waste of money. Playing on your paranoia, and looking for things to charge you for. "well it might fail on XXX, so we did you an estimate"...