m0n0wall Forum - WAN dhcp: dhclient lease time

I have m0n0wall set up with DHCP on the WAN interface.
This way, I am able to get a static ip from my ISP, but for some reason dhclient is always trying to get a new lease with different and (very) short lease times:

Oct 28 14:05:26    dhclient: bound to 81.*.*.* -- renewal in 2748 seconds.
Oct 28 14:05:26    dhclient: New Broadcast Address: 255.255.255.255
Oct 28 14:05:26    dhclient: New Network Number: 81.*.*.0
Oct 28 14:05:26    dhclient: DHCPACK from
Oct 28 14:05:26    dhclient: DHCPREQUEST on sk0 to 195.*.*.* port 67
Oct 28 13:52:05    dhclient: bound to 81.*.*.* -- renewal in 801 seconds.
Oct 28 13:52:05    dhclient: New Broadcast Address: 255.255.255.255
Oct 28 13:52:05    dhclient: New Network Number: 81.*.*.0
Oct 28 13:52:05    dhclient: DHCPACK from 195.*.*.*
Oct 28 13:52:05    dhclient: DHCPREQUEST on sk0 to 195.*.*.* port 67
Oct 28 13:30:43    dhclient: bound to 81.*.*.* -- renewal in 1282 seconds.

etc.

I am using 1.33 (built on Wed Mar 16 12:01:51 CET 2011) Generic PC platform.
The WAN NIC is an old (so I presume fully FreeBSD supported) onboard Marvell Yukon 88E8001 NIC.

Can someone tell me how I can disable these frequent DHCP renewals, e.g. by increasing the lease time?

Thanks in advance for any help

slth