"I don't know when I bought this watch."
Translation:Δεν ξέρω πότε αγόρασα αυτό το ρολόι.
Όταν has the meaning of "while" and is a conjuction of time ie it introduces a subordinate clause that specifies time. When I was there, that happened=While I was there, that happened= Όταν ήμουν εκεί, συνέβη αυτό. Πότε is when in the meaning "at what time?, how long ago?" and is an Interrogative adverb ie introduces indirect (as in the above sentence) or direct questions.