In the configuration.h file, you will find:

//#define PROBE_SERVO_DEACTIVATION_DELAY 300

I highly suggest you uncomment this line so that it detaches the servo after movement. You have probably noticed the servo jitters while printing if you do not have this line uncommented. This constant jittering will cause servo failure. Note that if you are using a slow servo, you may need to increase the delay.

If you look in marlin_main.cpp you will find two functions:

static void engage_z_probe()
and
static void retract_z_probe()

These are where the calls are made to extend and retract the Z-probe. You will notice they have if statements that look for a value of PROBE_SERVO_DEACTIVATION_DELAY. If that value is >0, the servo is attached and detached. The Arduino will only send a drive signal to an attached servo. This prevents the jittering.