I interpreted this one as expiration date, but . . .
To answer both of what Titchou's asked for clarification about
Expiration date: This will be found either on the original packaging of the inhaler, or (more likely) on the canister of medicine itself.
My rescue inhaler (salbutamol/albuterol in the states) is labelled as good for nine months, which it says on the canister. I can't recall the expiration period on FloVent right now.
How long its effects will last: depends what you're on.
FloVent has no immediate effects, and can take 2-6 weeks to begin working. The long-acting bronchodilator in Advair will peak in 15-20 minutes and last for about 12 hours (Advair also contains fluticasone, which is in FloVent.)
The effects of albutamol (albuterol, Ventolin) will last 2-4 hours, I think Xopenex lasts a bit longer, 4-6 hours.
If you have any questions about your meds, your pharmacist is a good person to turn to as well