It is too broad a question. "When you release the finger and return to the string" can only be answered by "it depends". There is immediate full release where the finger returns to the string, there is immediate release where the finger may not hover over the string it just played, but is not "held" away from the strings either. There is timed release where you may relax the finger that just played but not fully bringing it out over the strings. So there is no definite time that one MUST release the finger and return it to the string. Sometimes it is timed with the exchange between i and m or between a and i,m. Sometimes all fingers are released when p plays.
When beginners are taught, often they are taught immediate full release because many people hold a lot of dysfunctional tension in their hand and learning to release it all is the first step. The other releases are situation dependent.
Hope that helps.... maybe one of the teachers can elaborate on this explanation.
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