Alexodiy, как-то так?
<!DOCTYPE html>
<!-- © рони https://javascript.ru/forum/events/50724-vyvesti-znacheniya-value-iz-select-v-input-3.html#post471442 -->
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
window.addEventListener('DOMContentLoaded', function() {
var select = document.querySelector('[name="sitetime"]'),
res = document.getElementById('rez');
select.addEventListener('change', function() {
res.value+=this.value+','
});
res.addEventListener('input', function() {
var v=(res.value.split(',').filter(function(v){
return !!v.trim().length;
}).pop()||'').trim();
if([].some.call(select.options,function(opt){
return opt.value==v;
}))
select.value=v;
});
});
</script>
</head>
<body>
<select name="sitetime" id="sitetime" >
<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' />
</body>
</html>