In our environment, I have ours set at once a day between 9am and 11pm. We only really use policy delivery methods for the Software Deployment Portal and everytime you launch the portal it does a policy update anyways. For all other software distribution we mainly use multicast or push. I guess it all depends on your method of getting the software out to clients. If you're not doing a lot of distribution with a policy distribution method then I would not have it occur that frequent.
Edit: Our laptops are actually set to do a policy update at logon and when IP address changes as well. Mainly set it this way because we have a lot of laptop users and they go through our Gateway Appliance.
Our agents check for policy every 4 hours. As was mentioned in the last post, your users can manually trigger a policy check by opening the Software Deployment Portal (Desktop Manager). For my deployments, if I need the user to receive the package sooner than that, I will use a policy supported push. It allows all systems that are online to receive it immediately, and any offline systems to pull it as policy.
I prefer 4 hours, we have used this on most of our systems for the past few years and it works well.