They say always keeping $10k in cash is recommended.
Personally, If you're living below your means, I would try to target 6 months to 1 year. Then again, the past 3 years have made me paranoid as hell. Your mileage may vary.
FWIW - I sleep a LOT better knowing I've got a year worth of expenses sitting in a high yield savings account. If I lost my job tomorrow? I'd say fuck it and take at least 4-6 weeks off before bothering to look for a job.