GTN Spray effectiveness

I have been prescribed GTN spray as I have some symptoms that the doctor I saw earlier this week thought might be angina, he said to try it. Im not impressed with that approach, never mind though. I just wondered, does either someone who has angina or who knows about this know how long a puff of GTN spray will typically work if seomone does have angina and if the GTN works in that case? I understand response to GTN is not a diagnosis and that GTN can relieve GERD etc. Just wondered about the time it is effective for. I can feel the effect of the spray about a minute later - headache, head rush, fast heart thats about it for symptoms, no relief of my discomort.

Thanks