Hi Adnan,
1. In a general LMS/Backup LMS setup, AP will detect a controller to be down if it missed 8 consecutive heartbeats with the controller.
(Bootstrap threshold=8 in AP system profile defined it).
After missing these 8 heartbeats, it will try to switchover to backup LMS.
Depending on whether CPSEC etc is enabled, it will consume some time in bringing up IPSEC (CPSEC enabled), PAPI & GRE tunnels & then start serving the clients.
Note: The bootstrap threshold for RAP's & Mesh AP's is different.
b) If you are using the new HA feature, failover time can be reduced considerably as AP's will create a standby tunnel to the controller defined under backup LMS.
So, if primary controller goes down, it just activates its backup tunnel
You can make use of inter-controller heartbeats in the HA profile in case of master-local to speed up the process.
The user guide has explanation for the same.