She will challenge him if she thinks he is vulnerable. If the economy is solid and we are not engaged in any unpopular wars she may not. She also has to be aware that if she challenges him and loses, she can never do it again.
If she doesn’t challenge him in another 4 yrs. time she will be too old to run for office in another 8.So even if things are going smoothly , i am sure her liberal left wing media will cock something up and she will use that against him