Your tier status is valid for 12 months from your Petals Perks anniversary date. If the minimum spending amount for the past 12 months is not achieved, your tier will be downgraded automatically and your overall spending will reset to start accumulating petals again. If your tier is downgraded, your benefits change accordingly.
If you meet the spending requirements for the higher tiers, you will automatically be enrolled in the higher tier and unlock the perks accordingly.