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Check this solution : https://answers.ros.org/question/244311/how-to-detect-subscriber-drops-a-message/?answer=270198#post-id-270198 (the one at the bottom)

bottom) You can get more info here : http://wiki.ros.org/Topics#Topic_statistics

http://wiki.ros.org/Topics#Topic_statistics It give you a nice approach to detect if message are not received (by using TopicStatistics.traffic or TopicStatistics.delivered_msgs, by checking for how long the data remain the same).

same). I just discovered TopicStatistics, so I cannot give you any insight on how to use it (and how it affect performance, if it do)do)

EDIT: Topics statistics messages are send by the subscriber, when the subscriber received a message, hence it will not be usable in your use case (when the subscriber doesn't received a message).

2)

You can use some Python basic library to do the job (But you lose the awesome feature of topic statistic) :

import rospy
import threading # Needed for Timer
from std_msgs.msg import String

rospy.init_node("test")

def timeout():
    print("No message received for 5 seconds")
    # Do something

def callback(msg):
    global timer
    print("Message received")
    timer.cancel()
    timer = threading.Timer(5,timeout)
    timer.start()
    # Do Other thing

rospy.Subscriber("/test",String, callback) # When receiving a message, call callback()
timer = threading.Timer(5,timeout) # If 5 seconds elapse, call timeout()
timer.start()

while not rospy.is_shutdown():
    #Do something else
    rospy.sleep(1)

This code give you the idea, by using a timer you can check when no message are received. If a message is received, you reset the timer (By cancelling and starting a new timer). If no message are received, then you can do what you want in the timeout() function.