@sam sam , From your description, it seems we want to migrate from SCOM to Azure monitor. If there's any misunderstanding, feel free to let us know.
To migrate from Operations Manager to Azure, based on my research, there are no migration tools to convert assets from Operations Manager to Azure Monitor since the platforms are fundamentally different. Our migration will instead constitute a standard Azure Monitor implementation while you continue to use Operations Manager. As we customize Azure Monitor to meet our requirements for different applications and components and as it gains more features, then you can start to retire different management packs and agents in Operations Manager. We can see more details in the following link:
https://learn.microsoft.com/en-us/azure/azure-monitor/azure-monitor-operations-manager
For the time cost for set up on the Azure, I think it depends on our requirement. We can consult Azure-monitor support to get more help. I notice the "azure-monitor" tag is added. We can see if we can get suggestions on this. If not, we suggest to submit a new thread and only add "azure-monitor" to get right support.
Hope the information can help.
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