The hard limit is currently hard-coded to be 1 hour. This also determines the NN automatic lease recovery interval. Something like 20 min will make more sense.
After the 5 min soft limit, other clients can recover the lease. If no one else takes the lease away, the original client still can renew the lease within the hard limit. So even after a NN full GC of 8 minutes, leases can be still valid.
However, there is one risk in reducing the hard limit. E.g. Reduced to 20 min. If the NN crashes and the manual failover takes more than 20 minutes, clients will abort.