The survey length (LOI) is how long you estimate a person to fully finish the survey.
The time in field is how long you anticipate the survey being open for. This field is only to help estimate feasibility. This won't change, limit, or effect fielding in any way.
Incidence rate (IR) is the percentage of people that will qualify for the survey. In the screenshot below we are assuming 70% of people who enter the survey will be able to complete it.
Within the TapResearch tool, calculate your incidence after you've targeted. Proper targeting will provide a higher incidence, lower cost, and help your survey run more smoothly.
For example- If you are looking for respondents in New York City, the incidence will be very low if you target the US. However, if you target the New York DMA that will exclude the rest of the US and your incidence will be high.
(For more information on incidence, see here)