I can't speak for the 1099 on miles, I was curious how that would end up this tax year. Is there a definitive answer yet?
As for the bonus, it generally specifies 90 days from completion of required activity. So, say you open the account 3/1/13. You deposit the 50K for Citigold, and spend the $750 or whatever your offer requires, by 3/15/13. Often those promos require a direct deposit or a bill payment for 2 months, or something like that. Basically, it's within 90 days of the COMPLETION of activities. Once you get the miles, they're yours, and Citi won't claw them back. Even before you get them, you should be able to convert your account to Basic ($1500 minimum), and still get the Citigold level of miles.
tl;dr version: 3 months minimum, 6 months maximum, and yes you can convert to basic and pull your $50K back out.