The Chrono Treasure is a monthly event that lasts for 7 days

Complete daily and weekly tasks to earn Chrono Shards. Use these to progress through the floors and reach the final level to claim the final reward

Chrono Shards can be accumulated, they are not lost at the end of the event

To reach the final floor, you need a total of 8,240 Chrono Shards including puchasing the reward on the last floor

By completing all event tasks, you can earn up to 2,250 Chrono Shards for free during each event therefore, to reach the final floor and claim one final reward without spending money, you will need to save Chrono Shards for 4 months

Of course, if you want you can buy Chrono Shards to advance faster you can. I would only really recommend doing so if you have not yet unlocked the UR cards, else waiting is a viable option - the value of the packs drops significantly at the higher end