Well, first of all it's high time you realized you have to move on to Google Books (which shows books published and edited in English, thus providing a much more educated medium) from vanilla Google (which shows many non-native or uneducated examples).
Now, at Google Books:
"It's a long time that"
About 90 results
"It's a long time since"
About 279,000 results
ANY teacher and specialist would tell you based on the very low ratio here that the first is non-idiomatic and they would be right.
Now, this doesn't mean educated natives have never used this construct, see:
Letters of Lady Rachel Russell: from the manuscript in the ... -
Page 74 Lady Rachel Russell, Thomas Sellwood - 1809 IN my opinion
it's a long time that I have interrupted that commerce with which you have been pleased to honour me on your part, by my omitting to
make the due acknowledgments I ought for the packet of foreign
letters, and your own letter .
This is a doctor writing to Lady Russell in the 18C.
You might have seen somewhere this archaic style, and memorized it. And if so, good for you, but it doesn't make it idiomatic today.
Further, what this doctor says seems to me slightly different from the "since" construct:
*"It's a long time that I have interrupted that commerce."
seems to me to mean:
"That I have interrupted that commerce it's a long time."
or, further, very roughly:
"The fact that I have interrupted that commerce is equal to a long time."
which seems to me different from the process involved in "since" where we're looking along the time axis from a point on forward.
But this is just my speculation.
BTW, before researching this, I might have accepted your sentence, but I might have told you it's dialect or similar:-)