In open fields choice is determined by volatile action or molecules of any particulate matter emitted in the air. In open system insects usually take 20 seconds, but in cosed system insects hardly take 10 nano seconds which is clear from electro antennogram .

Preference can be determined by several means: for example: the time females take to lay the first eggs, the number of eggs laid during the first 24 hours after mating, by calculating the oviposition preference index, the total number of eggs laid on each host, etc. In this context, the amount of time you leave the insects depends of how do you measure preference. 

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In the case of controlled olfactometer studies the insects are generally observed between 5 and 15 minutes in the glass tube. If no choice is made in that time it is recorded as "no choice" and the observer moves on to the next insect.