Is encopresis a life-long condition or will it go away after some time?

Usually it does. While nothing in medicine is 100% certain, encopresis, if properly treated, will generally go away. Both the child, as well as the parents, tend to be just fine.
Depends on cause. Most cases dissipate over time. If there is a pathological condition causing the encopresis, then it's treatment would determine duration. The most common cause of encopresis in childhood is untreated adhd.