More than likely there will be no 'set period of games'. He'll be getting assessed continuously by those above him, and they will be approaching other managers in the meantime and seeing where the land lies.
If Tuchel wanted the job for example, I don't think it would matter if Unsworth was given 2 games or 20 games. The job will be Tuchel's. And conversely if Eddie Howe's the best we could get, and Unsworth doing well in the meantime, they'll probably let him keep it.
The board are right not to put a set timeframe on things. No reason to limit themselves in anyway. Assess the situation on an ongoing basis, put the feelers out there and see what their best option is over the next few weeks.
Stop talking sense, fly off the handle more... this is the Everton forum after all.