It really depends on the flavor(s). If it's a mix of flavors (like Strawberry Lemonade) I would say that the taste changes for a month, maybe 3 weeks. But single flavors should settle after 2 weeks. Note: I steep with the cap on, I've heard if you leave it completely open it speeds up the steeping process.
Although I've run into a few flavors that taste the same from mailbox to last drop. These are only my experiences, hopefully someone will pop in and correct me if I'm wrong.