If you have searched the article databases and start to see the same articles over and over again, then you have done your due diligence and can consider your lit review complete. That isn't to say an article might not slip through, but if you have done the steps below, then the chances of a really important article slipping past you is pretty slim.
Searched all relevant databases, using a variety of keywords and subject headings.
Mined article bibliographies for their cited references.
Looked in Google Scholar (finds articles, books, conference proceedings, etc) or Web of Science (finds scholarly articles, weighted towards the sciences) to see who has cited those articles.