Weekly challenges provide 3.2 tiers of progress in XP. Season 12 is 13 weeks long. So, 13 x 3.2 = 41.6 tiers. Each Rocket Pass has 4 stages. You gain 7 tiers when you complete one stage. So, 7 x 4 = 28 tiers. Max tier in Season 12 is 285. So, 285 - (41.6 + 28) = 215.4 remaining tiers. That gives us 215.4/13 = 16.56 tiers a week or 2.36 tiers a day. You can also earn additional tiers by completing seasonal event challenges, such as the upcoming Halloween event. Thus, you should be able to unlock all tiers in Rocket Pass 12 by achieving 2 tiers per day.