If you are still paying off debt, then you should have about $1000 in savings and put all you can towards non-mortgage debt. If you don't have any debt besides your mortgage, then add up all of your monthly expenses including food, gas, utilities and keep 3-6 months in liquid savings.
Whether you keep 3 or 6 months depends on how safe your income is. If you have a steady safe job, you might be safe with 3 months. But, if your employer is cutting back or you are in a commission based job or self employed - then lean more towards 6 months expenses.
I suggest you to please go through this article about Importance of Emergency Funds