As a benchmark, some chartists use a 3% penetration as a criteria, particularly for major support and resistance levels. Shorter term support and resistance areas would probably require a much smaller number, like 1%.
In reality, each analyst must decide for himself or herself what constitutes a significant penetration.
It’s important to remember, however, that support and resistance areas only reverse roles when the market moves far enough away to convince the market participants that they have made a mistake. The farther away the market moves, the more convinced they become.