[рони]

<select name="sitetime" id="sitetime" onchange="document.getElementById('rez').value = [].map.call(selectedOptions,el=>el.text)" 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' />