By default, Update manager should check less often for updates

I use Linux Mint 9 LXDE (nice distro!) on an old computer. By default, Update manager checks every 15 minutes for new updates.

This slows the old computer down a lot. The system nearly freezes every time. So I had to increase the interval for the update check to once a day.

The default interval for the update check should be once a day, which is good enough for almost everyone.