try this
<p id="para1"> Your text </p>
<script>
var text = document.getElementById('para1').innerText;
var newText = '';
var count = 0;
for (var i = 0; i < text.length; i++) {
newText += text[i];
count++;
if (count == 100) {
count = 0;
newText += '<br />';
}
}
document.getElementById('para1').innerHTML = newText;
</script>