HER2 is a protein that when defective (i.e HER+) causes breast cells to divide and multiply uncontrollably. Herceptin targets the HER2 receptor in those cells blocking the ability of HER2 to bind with new cells. This typically results in slowing cancer growth or decreasing the risk of recurrence in adjuvant therapy for those already in remission.
The standard protocol for herceptin is one year, but there are some new studies underway to determine if that should be more or less. Since herceptin is an antibody, resistance to its effects are expected to decline over time and generally the risk of heart damage increases over time.
In most cases, metastatic cancers are considered treatable but not curable. Hence, treatment becomes ongoing.
In order to get another opinion, you may need to have something a little more substantial to go on. Getting additional medical opinions is for the purpose of getting additional input on medical procedures or treatment plans, not comparison shopping for doctors. So unless your new oncologist has already developed and provided a treatment plan for you, there really isn't anything to get a second opinion on. However, if you have a written treatment plan from your former oncologist, you could take that to a different oncologist (including your current one, if that has not already been done) for a second opinion.