In Netherland, the standard trial period is a month, during which you can be fired at any time for anything. I'm not sure it's legal to extend that trial period, but after that, people usually get a half-year contract, which won't get extended if it turns out they suck during that half year. There's a strict limit to how many temporary contracts you can get. Eventually, the employer has to offer a permanent contract. You can't let the employee live with that kind of uncertainty indefinitely.