The time displayed for each survey is an estimated average time frame. There are many variables such as internet speed, reading ability, etc. that might make the time you spend on a survey a bit longer or shorter.
When participating in a survey, the number of questions you receive is based on your responses to previous questions. As a result, you might complete the survey after answering a few questions, while another participant may need to answer a lot more before finishing.
The LifePoints (LP) allocated for each survey are fixed and cannot be changed, regardless if you took longer to complete the survey than the average time indicated.