Free IQ tests can often be completed in 30 minutes or less, while paid IQ tests can take much longer. This is due to the fact that paid IQ tests involve more complicated assessments using scientific methods, and they yield more accurate results.
Free IQ tests are often multiple choice for ease of administration and can thus be completed quickly. They also give bonuses for finishing them quickly, unlike more scientific methods of data collection. The only way to get an accurate IQ test is through a psychologist or other qualified professional. A paid IQ test is generally more acceptable as an official indicator of intelligence.