nobody can give you a accurate time frame there are a lot of factor that determine that including but not limited to employee turnover, retirements, and whether casual or PT cover can be used.
You still can be put on the no rehire list. Each driver gets ask about there helper and if UPS should ask then to be a permanent employee or not the same goes for seasonal working on the inside of the building. I have witness a seasonal employee set offered a permanent PT position one day and the next day get told he not need after today when he didn't have to do what he was told.
By the way your first post is wrong you have been working for UPS for less than a month not 2, you must of forgot about your Post on Dec 1 when you where still waiting for a call back from UPS. With that being said your limited to having worked for UPS for 15 days if they called you Monday morning and told you you where working Monday night. We both know that didn't happen.