I have read that a hemoglobin level below 5 g/dl is very dangerous and it can be fatal how long can some one survive with a hemoglobin of 5 g/dl my wife hemoglobin is 6.5 g/dl she eats very less food how serious is the condition?

Yes, such a low hemoglobin is dangerous, not necessarily though the actual value 5 or 6, but the danger is created by the speed at which the hemoglobin is dropping. It has to do with stopping the bleed or treating the condition, that may be serious that is causing the anemia or low hemoglobin.

Her anemia is likely due to menstrual blood loss. She should take oral iron and multivitamins. Your doctor can also give an infusion of iron to correct the deficiency.