How long is considered "prolonged use" of artificial tears? 1 week, a month?

Opinions will vary. "prolonged use" is a subjective term and there is no clear cut answer for your question, so this will vary from physician to physician. For me, if you need artificial tears for a month, then there is a problem that should be investigated. If it's just a few days or even 1-2 weeks then its likely not a problem.