The systemd service unit option that controls the timeout is TimeoutStartSec:
# Configures the time to wait for start-up. If Jenkins does not signal start-up
# completion within the configured time, the service will be considered failed
# and will be shut down again. Takes a unit-less value in seconds, or a time span
# value such as "5min 20s". Pass "infinity" to disable the timeout logic.
#TimeoutStartSec=90