How often should i go for a lung function test if i'm a chronic asthmatic patient?

Depends: Many with mild asthma never need to have them done. Those who have stable asthma responsive to simple daily meds with no ER visits may be managed without at the discretion of the consulting physician. Those with unstable asthma may need them on a regular schedule, depending on findings with each study. Asthma is such a variable disease a single answer would be unreliable.