laimas,
<select name="sitetime" id="sitetime" onchange="document.getElementById('rez').value = [].reduce.call(options,function(a,b) {
b.selected && a.push(b.text);
return a
},[])" 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' />