The answer is, an hardware limitation.
Simply doing l2vpn capability we can see the aging time "limitations":
RP/0/RSP0/CPU0:9K10-4-512-RGA#sh l2vpn capability | in Aging
Tue May 10 13:18:39.883 CET
VPLS MAC Aging Default Timer Value: 300
VPLS MAC Aging Min Timer Value: 300
VPLS MAC Aging Max Timer Value: 30000.