The standard Wilderness First Aid class is 16 hours. It does not include CPR which you should take separately (and may be able to get free through your employer). It is well worth the time and money! You can, of course, take a Red Cross standard first aid course, but those classes are based on emergency services being right around the corner. In a wilderness situation, rescue may be days away. In a WFA class, you will learn about evaluating and stabilizing a patient, how to move a patient, when rescue is necessary and when you can walk the patient out, how to improvise with materials at hand in the field.
REI has been offering Wilderness First Aid classes at many of their stores, mostly through NOLS.
Here's the NOLS schedule, which includes the classes they teach for REI: http://www.nols.edu/portal/wmi/courses/wfa/
In some areas, the American Red Cross offers WFA for about the same price. They require you to have current CPR/AED certification before taking the class.
My WFA class was through REI and taught by Remote Medical International; their classes are offered mostly in NW Washington, although they did teach in the Portland area last spring. Here is their schedule: http://www.remotemedical.com/wilderness-medicine-training/Wilderness-First-Aid-WFA