The connotation is just like what Tang said. I suspect people coined the phrase 間唔中 as a result of finding an alternative to 耐唔耐. Here the first 耐 is in tone 6 as usual, but the second 耐 is in tone 2. This phrase means "a long time or not," and is written as 久不久 in written Chinese. They are now interchangeable in daily conversations.
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