Need help with miniupnp lease duration

Hey guys
I have a WRT1900ACV2 with latest LEDE trunk and luci-app-upnp(includes miniupnpd) installed.
It seems that miniupnp gives each client a maxium leaseduration of 604800 seconds like this: miniupnpd[2293]: AddPortMapping: ext port 49250 to protocol TCP for: Minecrafter_0200(DCDN) leaseduration=604800 rhost=

And after 7 days(604800sec), miniupnp remove this port mapping automatically:

daemon.notice miniupnpd[1602]: remove port mapping 49250 TCP because it has expired

My question is how can I change the leaseduration to permanent? I couldn't find such options in luci upnp GUI.