When can we stop testing?

This depends on number of factors:
1)In case all the planned test cases have been executed
2)In case deadlines are over
3)Defect density(Number of Bugs/Lines of code) reduces
4)No more bugs are found with execution of test cases
5)Alpha and Beta Testing are over

Test Strategy

Test Strategy:
Test strategy is a high level static document which is not subjected to changes or modifications again and again.This document is created to cater Business requirements and needs.Test plan takes it contents from Test Strategy only.
In some cases test plan includes test strategy but that happens in case of small projects for bigger projects Test strategy is the main document that contains:
1)Scope
2)Risks and mitigation plan
3)Test metrics
4)Test deliverable
5)Industry standards to follow
6)Business requirements to cater