"Retest effects in cognitive ability tests" by Scharfen & friends is a good overview that mentions other studies that go into more detail and tweak different parameters (e.g. time between tests, tests that are more repetitive than others, etc). The meta-analysis found that retaking a test can add a couple points each time. So taking an IQ test 4 times on average yields an 8 point increase, almost an entire standard deviation.