France : this is all regulated. The law says that
- you have a certain amount of days off (varies by job, in my case about 40 days a year)
- you have to take 10 days in a row (2 weeks) during the "hot season" (June to September IIRC)
Beside that this is an agreement between you and your boss. I guess it would be illegal to formally set a cap on continous vacation, in practice it may happen but I've never heard of that.
People will typically take 2 to 4 weeks during summer (with an average of 3). In smaller companies one can be forced to go on vacation in August when the company closes.