I don't remember seeing it offered to new hires in class but my guess would be after the first vacancy bid after you're hire date. Vacancy bids have been happening every 3-4 months. Flow to new base would be a few months after that.

Signing bonus: If you leave within the first year, you are supposed to pay it back in full. Between the end of year 1 and year 2, the amount is prorated. You're clear after year 2. You will sign a contract stating such if you come here and accept the bonus. I have not heard if the company is legally persueing it's legal right to bonus payback for those that leave in less than 2. Really, I find it odd that no story is being told here on that.