Test Execution is one activity which is most difficult to estimate. Most of the time it happens that if testing team find big chunk of bugs in the software then frequent releases/builds are given to testing team due to which testers face time crunch in test execution. This might results into the skipping of *critical* bugs during testing if high priority areas are not well tested.
To avoid these situations, Time-boxing should be achieved. In Software Testing, Time-boxing can be achieved by doing Risk Analysis.