The WPA textbook edited by Norman Sartorius and Mario Maj says after 2 years without symptoms for patients with a single episode, and 5 years symptom-free after multiple episodes.

Thank you, I know these guidelines, but there are some literaturę data, that withdrawing antipsychotics from patients who have been stable for up to 6 years leads to more relapses than continuing medication. See meta-analysis by Leucht:

http://www.ncbi.nlm.nih.gov/pubmed/21208500

To be absolutely safe from relapse, probably never. But if you withdraw very gradually and slowly and then always keep pills in your pocket IN CASE, it should be relatively safe.