It probably depends on what version of manila you are using. I'm on sense 2.5.1921 and it has the following in the registry:
Code:
[HKEY_CURRENT_USER\Software\HTC\Manila]
"Weather.AutoDownloadInterval"=dword:3C
The 3C is hex for 60 minutes. You could change it to 1E for 30 minutes and see if it works (assuming that registry key is part of your manila). I do not see any similar options in the registry settings for an older version of manila 2.1 (pre-sense UI).