Perhaps a better question would be how long could a modern commerce raider operate?
Any commerce raider could operate at least once.
Also, you should define what you mean by commerce raider. When I think of commerce raiders I don't think of Somalian pirates. I am thinking more along the lines of the Emden. Since you are asking the question, how do
you define commerce raider?
Also if a commerce raider's definition includes being controlled by a state government, how would it operate without a declaration of war or a state of OOTW existing?
Or are you thinking of a terrorist type of commerce raider?
It is tough to answer this question without some bounding of the issue.