If the content in your module is fixed or static, you can set cache for long periods. When modules have frequently updated content, then cache should be very small or none at all. The settings in between are experimental, but remember, more cacheing is likely to speed up your site's performance, but should not restrict content changes.

There is an excellent article on this topic at wolf factory. you can link to it here

This is of great help - it certainly will help stop us putting cahing on were it shouldn't be!