Current lock stretch of green laser to X arm is limited by ~ 30 mins.
I found that the lock loss is probably due to the limited range of the frequency actuator, the VCO (see 6544).

This klog tells us the linear range of the VCO input is between ~0.8V to 2.8V.
And the attached playback figure show that the lock was lost when the feedback signal to VCO got close to 0.8V.
To have a longer lock, we need to implement a slow loop with test mass suspensions.
It is not going to be hard since it is OK even if it is slow/.