How to compute the residence time for given flow rate in standard cubic meters? I'm given a flow rate of a gas mixture of $3\ \mathrm{Nm^3/h}$ in standard cubic meters through a cylinder of length $50\ \mathrm{cm}$ and cross section $70\ \mathrm{cm^2}$, can I compute how long a particle will stay inside of the cylinder (which would equal the time the particle can react inside the cylinder)?

And what if the temperature changes during the reaction?

If the flow rate would be given in volume per time, I had an idea to compute this, but the use of standard cubic meters is baffling to me.
The N stands for "norm" and should be avoided (in fact, the German Wiki page lists the Nm³ as wrong abbreviation) as it can create confusion.(qed) 

So your flow should be written as "3 m³/h under standard condtions". 
If you can guess German, have a look at the German Wiki page. It is much better than the English one.

Standard conditions vary a bit between different norms 

T = 0 °C or 15 °C or 20 °C 
p = 1.01325 bar (sometimes 1 bar)

The gas is supposed to be dry.