f5 will mark down the back end servers and 15 minutes later mark up so we will be monitor the back end servers periodically like down and 15 minutes later up and 15 minutes later down.what are you going to do when traffic is coming but pool is down?
besides external monitor, i think irule can make it too by calculating on time (e.g. mod). the following is discussion in the past. although it is not exactly what you are asking, it might be helpful.
Schedule iRule
https://devcentral.f5.com/community/group/aft/1179351/asg/50
Schedule an iRule for specific date/time
https://devcentral.f5.com/community/group/aft/13201/asg/51