The extent of testing required for a software product or feature depends on several factors that influence the level of quality and risk involved in delivering or deploying it. Some of these factors are:

Budget to do testing: This factor indicates how much money is available or allocated for conducting the test activities, which affects the scope, depth, duration, and resources of testing.

Time available to do testing: This factor indicates how much time is available or allocated for conducting the test activities, which affects the schedule, frequency, speed, and coverage of testing.

Level of risk of the product or feature: This factor indicates how much impact or harm a failure or defect in the product or feature can cause to the users, customers, stakeholders, or environment, which affects the priority, intensity, complexity, and rigor of testing.

Complexity of the product or feature: This factor indicates how difficult or challenging it is to understand, design, implement, or maintain the product or feature, which affects the effort, skill, technique, and tool required for testing.

A particular tester involved in testing is not a deciding factor in determining the extent of testing required because it does not affect the quality or risk level of the product or feature being tested. Rather, it is an outcome or consequence of deciding how much testing is needed based on other factors such as budget, time, risk, and complexity. You can find more information about determining the extent of testing in Software Testing Foundations: A Study Guide for the Certified Tester Exam, Chapter 2, Section 2.12.