What are optimal settings for Recycling of Application Pools in IIS7 in a shared environment?
If you have a heavy traffic site, use long recycle schedule. If you have a low traffic site use shorter/default schedule to save memory.
I learned this from Al Zabir's blog: http://msmvps.com/blogs/omar/archive/2008/10/04/best-practices-for-creating-websites-in-iis-6-0.aspx
Daniel S. is right, your session variables get destroyed on recycle, so make sure you test this well or have good error protection/recovery when getting your session objects.