The current situation is that by law you can give a maximum of 3 temporary contracts, during a 3 year (36 month) period and to break this row there need to be 3 months between these contract.
From July 1, 2015 under the new employment rules 2015 the situation becomes:
You can give 3 temporary contracts, during a maximum 2 year (24 month) period and to break this row there needs to be a minimum of 6 months between these contract.
So the contract will be indefinite when a period of 2 years has lapsed or the 4th contract is provided.
The best minimum contract is a 7 months contract as you then have the probation period. The best maximum contract is a 23 month contract as you then have not transition fee to be paid should the contract not be renewed.