It can be thirty days or any time period. Most are ninety days. You can get a good idea within three months if a new employee will work out or not. Tell your prospective employee in so many words what duties you have in mind for them and ask if that is in line with what they were expecting from the job. And verbalize those expected duties.
Every applicant has one thing on their mind.
How much salary they can push for up front. Most will ask for more money. The burden is on the applicant to prove to you the employer that he or she is worth their salt. You and the applicant each have an idea of how much the salary will be. If the applicant has decided beforehand that they can live with a certain salary with the promise of a raise and you the employer come close to meeting the salary expectation, then a deal will be struck.