Hi Hobbit,
When using set flow-stat interval command, if you set all the working interface as 300 seconds, the CPU is in the normal range. When you set all the working interface as 60 seconds, the CPU usage maybe a little increase, but it won't increase much.
We are unable to provide the exact CPU value that will increase as this depends on the traffic on the live network and the number of the UP interfaces.
I hope this helps.