For asthma cough, how effective is Floven inhaler 250mcg 1 inhale twice a day?

It prevents flareups. Flovent is a 'preventer' medicine -- it will help prevent asthma flareups but isn't as effective in helping if the flareup is already happening. If you're having asthma symptoms right now, please see the Dr. -- you need 'reliever' medicines like albuterol and maybe prednisone.
Helpful. Flovent, an inhaled steroid, can be very helpful in reducing the inflammation (swelling) brought on by asthma. This medication can be used not only for the treatment of an attack--in association with albuterol--but also as a maintenance (preventative) after the attack has been controlled. When inhaled properly, very little gets into the circulation, so negligible systemic effects.