How to obtain the longest character with "：" and the offset value of the first character after "："?

Select the text frame with the colons (or select a few lines), then…

app.findGrepPreferences = null;
app.findGrepPreferences.findWhat = ': \\K';
x = app.selection[0].findGrep();

max = 0;
for (i = 0; i < x.length; i++) {
  max = Math.max (x[i].horizontalOffset);
}

alert (max);