The time consuming effort in the upgrade would be
1. CMDB upgrade
2. Moving over the customizations in the KB of the customer's cell to the BPPM 8.6 KB. There is an utility but customer will need to make sure the customizations are moved over properly. This activity may take time based on how familiar the customer is with the KB. Also it's a crucial task.
3 Cluster setup in case customer requires HA (active passive). If the presentation SIEM cell is not in HA then it's easy, as Cluster setup will not be required.
The time for upgrade will depend on the current deployment and the intended target deployment of BPPM.
Can you share the current SIEM deployment? Based on that I will be able to comment in detail.