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Hi, I have a question in my mind for a long time-----
How to say sb. came just very in time when you need help?
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The best expression to choose would depend, as usual, on the context in which it would be used.
Anyway, here are a couple of expressions that might help:
"You came just when I needed (your) help most."
"You came at the right moment; I was in need of help."
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OK, Miriam’s covered it. Maybe a less formal approach could be
“you got here in the nick of time”
I read Disney comics – use at your own risk!
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Comments  
Now you make me think of a question: 'Could you get here in the nick of time?'. Would this be an appropriate question?
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Do be punctual darlin'