First, I commend you for providing such thorough information. The answer is actually quite simple: Yes–90 days
Explanation: As in all jurisdictions, in Alberta, temporary or probationary employment becomes permanent after a certain period to the extent that an employee’s rights to termination notice and other benefits under the ESC vest. In AB, that period is 90 days of employment. For the first 90 days, you can terminate the employee without notice for lack of suitability, which is a far less stringent standard than the “reasonable cause” that applies to permanent employees.
Possible Exception: ESC requirements are minimum standards and employers can provide more generous terms if they want to. In this case, your company (or offer letter) may stipulate that the probationary period is less than 90 days.
Advice Going Forward: Have each temporary employee you hire on a probationary basis with the prospects of full-time employment sign a probationary employment contract stipulating that the employment IS probationary for 90 days, that lack of suitability is grounds for termination during the probationary period and that ESC rights and benefits don’t vest unless and until the employment does become permanent. Here are some excellent resources from HRI: https://hrinsider.ca/the-7-things-you-need-to-know-to-make-probationary-employment-work/ AND https://hrinsider.ca/model-policy-on-probationary-employment/ (based on Ontario but can be easily adapted for AB)
Hope that helps. Glenn