Every 3 days you will be able to try and unlock a Quest Orb. In order to do this, they have to finish quests from which they get quest points.
You get a new quest every 4 hours. You can have up to 3 active quests.
You have to enter a quest in order to get a new one.
After the 3 days run out, progress on quests and orbs are discarded and you are immediately given 3 quests.
If you happen to achieve a Milestone when completing your Quest and you don't claim it before the Quest's expiration, you will be given the reward to claim in your inbox.