Оно?
<!DOCTYPE html>
<html>
<head>
<style>
button { margin:4px; cursor:pointer; }
input { margin:4px; color:blue; }
</style>
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
</head>
<body>
<input type='text' id='rez' />
<select name="sitetime" id="sitetime" onchange="document.getElementById('rez').value+=value">
<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>
</body>
</html>