When does a legato touch become non-legato? By definition, whenever a small break is perceived between two tones, then their connection is non-legato. But in actual application there is more than one type of legato, and there are many variations of non-legato touch. To identify precisely when an articulate legato becomes a gentle non-legato is impossible.
A related question appears: When does a non-legato touch become staccato? This is at least as difficult a question to answer as the first. Since there are varieties of both non-legato and staccato touches, some could qualify as both a more detached non-legato touch and a less detached staccato touch.