It all depends on the context.
As a general rule, it is suggested that commas should be avoided wherever possible. This helps to make a passage more continuous.
The best way is to consider how you would say it out loud. If you find a section of the sentence where you pause whilst reading then this is usually where the comma should go.
Therefore, read the sentence out loud before you add the comma.
In the sentence above, I had to pause after therefore.
But, for example, in the sentence:
He was short and therefore unable to reach the book on the top shelf.
The comma was not required because I didn’t have to pause after therefore.