What can cause DBCC CheckDB to take longer than usual?
Lots of options, kind of hard to speculate:


Slower IO to disks, perhaps due to activity elsewhere on the SAN?
Other activity on the box interfering (user activity, maintenance jobs, other databases, etc.)
Memory or CPU pressure on the box (admin logged on locally leaving things running, big SSIS packages eating up CPU or local memory)


Narrowing down which of these it might be won't be easy, you'll probably have to turn on some long-running perfmon logs and do some trending over time.