Same in Poland — after 2y on a permanent job contract the leave is 3 months. It goes both ways. I can be shorter if both parties agree.
Yes, if both parties agree, you can leave at once, otherwise employer can sue you for damages or lost profits (if damages are small, they don't bother) and you don't have unemployment benefits.
But this is for when both of them can't find a mutual solution. You can always agree to terminate the contract earlier, if both parties agree.
At least that is how it works in Germany.