"Approx" is a shortened form of "approximately," and as an abbreviation, it should follow the same rules as other abbreviations. You should not put a full stop after it unless it's at the end of a sentence.
For example:
* Correct: The meeting will start at approx. 10:00 am.
* Incorrect: The meeting will start at approx.. 10:00 am.
Instead of using "approx," it's generally preferred to use the full word "approximately" in formal writing.