When she applies (wherever, if she has legal status in Canada she could apply at a consulate there), there will be a timeline. Others can help you find it.
Your question however is whether she should apply now, or later. If she applies now, the TD visa will only be valid for a few months. If she waits until you have new 3-year TN, then her visa will be good for 3 years.
So it depends on how soon you want her to be with you in US, and how much she will want to travel after the shorter TD would expire.