And if anyone else is interested, here's my c# translation:

int j = 0;
while (j < sText.Length)
{
if (j + 250 <= sText.Length)
sPart = sText.Substring(j, 250);
else
sPart = sText.Substring(j);

txtBox.TextFrame.Characters(j, missing).Insert(sPart);
j += 250;
}