My white blood cell count has been very very low for many months now. is it serious ?

It may be: Very low blood count especially the fraction thereof called neutrophils can be a problem because it leaves your body unprotected against certain infections. Knowing #s would be better than just saying low or very low.

Depends : It all depends on the cause and the degree of the decease in wbc. Some ethnic groups tend to have mildly lower WBC at all time and that is normal. However, you doctor should search for alternative causes before assuming so. Medication and autoimmune diseases are common causes of low wbc.