IRule for redirection after several hours of downtime

you could try setting a timer using table in the LB_FAILED. You will have to remove the table entry if it exists in a LB_SELECTED event.

when LB_FAILED {
    if { [active_members http_pool] = 0 } {
        if { table lookup -notouch "[virtual name]_fallback" > 0 } {
            if { expr [clock seconds] - [table lookup -notouch "[virtual name]_fallback"] >= 7200} {
                HTTP::fallback 
            }
        }
        else {
            table add "[virtual name]_fallback" [clock seconds] lifetime -1               
        }
    }
}