A long session, 22 minutes: Resting state 9 minutes, and then a blocked design (2 minutes task + 30s rest) 13 minutes. The questions are:
(1) Is the head motion a concern?
(2) Can the 9-minutes rest be used as a baseline for the data analysis of the blocked design?

My answer:
(1) Yes. There may be more head motion for such a long session. But occasionally, a long rest is necessary (e.g., epilepsy and sleep study).