Both are correct, one is in the past simple and one is in the present perfect. Assuming that the word fresh was supposed to appear in both sentences (otherwise that would be a difference). It's been a long time since I had fresh strawberries - This is a statement about the past It's been a long time since I've had fresh strawberries - This is present perfect so it is talking about things in the past, but they are being thought about/ have an effect on the present.
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