You must keep 15k for 30 days, and then it takes about 90 days for the bonus to be paid. That article recommends downgrading the account, and then closing it after another 30 days.
If you don't keep 10k for the 90 day waiting period, you have to pay a $25 per month fee. [size=78%]So really, it's 15k for 1 month, 10k for another 3 months, and then wind down over the course of the next month.[/size]
This is one of the easier and better sign up bonuses I've seen.