Is this correct: "Let me know once you finish"? [closed]

Please help me. Which of these is correct?

1. Let me know once you finish

2. Let me know when you finish

3. Let me know when you have finished.

Solution 1:

Here is the Google Ngram, courtesy of @Peter Shor (slightly modified):

Based on this Ngram, I'd say that "when" is clearly the preferable adverb in this context. While I don't believe the first sentence is grammatically incorrect, it's awkward.

Sentences two and three are idiomatically correct - that being, widely accepted and natural sounding.

Solution 2:

It of course also depends on whether it's American English or British English:

British: American: