What part of speech is the word so in phrase 30 minutes or so?

In the phrase "30 minutes or so", the word "so" functions as an adverb.

Here's why:

* It modifies the meaning of "30 minutes": "So" indicates an approximate amount of time, making it close to but not necessarily exactly 30 minutes.

* It answers the question "how much?": "About how long?" – "30 minutes or so".

Therefore, "so" acts as an adverb modifying the noun phrase "30 minutes".