In the krb5.conf that gentoo ships with heimdal, ticket_lifetime is set to 600. I think whoever set this value though that this meant 600 minutes (or 10 hours, the standard lifetime for kerberos tickets), but in fact it means 600 seconds, which is a tad short :)
No, it is expected people run some daemon which can renew their tickets and tokens, for example when using long-time batch jobs accessing data residing on AFS partition. This protects people from having left their 10hrs valid ticket files in /tmp. You may change the default value by tweaking the default@REALM principal from kadmin(1). Or run "kinit --renew". Should be resolved as INVALID I believe.
Wow Martin, you and I agree for a change :P Henrik, I'd rather leave this as an exercise for the sysadmin.