Well, it depends on a number of things: the kind of organization you teach for, the type of course, the information you teach, the amount of time provided for each unit, whether the units are related, students' level and abilities, etc. If there are, say, 10 units in total, I'd do a review test every second lesson so as to get a feel for what needs to be reviewed further, then, once I am confident the students have a fairly good understanding of the units, I'd move on to the next two units and so on. After the 5th unit, I'd give a mid-term test dealing with units 1-5. Then I'd review any material students had a hard time with on the mid-term before starting on unit 6. Then I'd go back to giving review tests every two units, and then after the 10th unit, I'd give a final test dealing with units 6-10. Aside from tests, I'd also assign (group) projects and home assignments.
Note, if you are teaching overseas at a conversational school, tests may not be required by the organization you facilitate for, so you should check with them. One last thing, testing doesn't mean you have to give paper tests. You could always test your students in the form of a review or warm-up activity every single class if you'd like. Testing is about finding out what it is that the students know and don't know. Finding out what they know and don't know will help you meet the needs of your students more effectively when planning your next lessons. Testing is about helping students. It's a tool that measures how well your teaching works for your students.