Local GUP---------(if failed)-----------SEPM----------(if failed)----------Symantec webiste
1. The option "Maximum time the clients try to download updates from A GUP before trying the default management server" is a first failsafe (Local GUP---------(if failed)-----------SEPM) - client not reaching GUP for any reason will ask SEPM for updates after the specified time passes (you can set it up in minutes, hours, days). For obvious reasons days does not really make much sense and is least secure.
2. SEPM----------(if failed)----------Symantec webiste - > there is no direct setting like that in the policy. You can achieve that by enabling additionaly "Use a Liveupdate Server" in the same liveupdate policy. So alltogether to cover whole scenario you would need to enable:
- use the default management server
- use a liveupdate server -> default Symantec Liveupdate Server
- use a group update providerd
+ set - > Maximum time the clients try to download updates from A GUP before trying the default management server"
+ enable the Internet Liveupdate schedule for clients to check with internet Symantec Servers at specified times -> in this scenario Symantec website is not 100% failover to SEPM but the client will check both sources for update (SEPM according to heartbeat, Internet according to schedule).