Testing can sometimes never be fully completed, so software is released that may contain bugs. If every software was completely 100% tested it might never get released. The two easy ways to know testing is over is if the deadline is met or the budget is exhausted. The test coverage should be 95% passing, and the test coverage should be at least 95%. All major defects should be fixed so the 5% failing tests should be small defects.
Knowing when testing is over can be hard, but these few tips helped me learn when to stop.
Software Testing 