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Templates_c holds generated cached templates from smarty. deleting forces fresh versions to be generated including your changed content.
The folder is found in the root of your XOOPS file distribution and should be permissioned for 0777
How often, frequently after template changes, theme changes, updates etc. Not so often on a stable site.
Cant really give you times on this one.
Do a search here for more info
Re site... well, we'll just have to wait till your ready for a public launch. Post here in the news, we all love to have a peek around other people's site
You'll get some useful feedback too...