When does the testing takes place?

Dear Community,

iam wondering when exactly the testing takes place during the sprint. Of course the Increment should be releasable and meet the DoD and therefore tested.

But there are two aspects arising in my mind:

1) What if the testing outweights the actual development work and therefore hinders the progress of building the increment. More specific: Is there are limit for testing like it is for Product Backlog refinement?