I don't know how it is in France, but IIRC in NA personal cheques are good for 6 months after date of issue.
Also, even if they let that deadline expire they can still ask you for new payment of whatever you owe them for a period of ~2 years (again, in NA, and again IIRC), even though not cashing your cheque was their fault. I'd make sure that you keep the carbon copy of the cheque you wrote as some sort of evidence you gave it to them in the first place (to make sure that they don't try to charge late fees in case they ever notice they didn't collect your money).
In other words, they can decide any day to cash that cheque, or ask for a new one. Best to keep the money in your account. I bounced a cheque by mistake one time and it wasn't fun. There were no real consequences beyond a 25$ fee, but I felt like a complete deadbeat when I had to explain it.