Thank you for this question,
Pruning arborvitae in the landscape is seldom required due to their dense growth habit; however, they can be pruned at any time, with early spring being preferable.
It is recommended to not expose any of the brown interior, that area may not recover or will do so slowly. Depending upon the source, they can live 25-50+ years.
- does not recover from severe pruning (where the bare stems are exposed, although side branches may slowly envelope the dead stems)