[рони]

"старый добрый ..."

<script>
   function fn(sel)
   { var a = [],i = 0, b;
     for (; b = sel.options[i++];)  {if(b.selected) a.push(b.text)};
     document.getElementById('rez').value = a;
   }
</script>

<select name="sitetime" id="sitetime" onchange="fn(this)" multiple="multiple" size="7">
  <option value="" ></option>
  <option value="1" >60</option>
  <option value="2" >90</option>
  <option value="2" >120</option>
  <option value="3" >150</option>
  <option value="3" >180</option>
  <option value="4" >210</option>
  <option value="4" >240</option>
  <option value="5" >270</option>
  <option value="5" >300</option>
</select>
<input type='text' id='rez' />