VAR occupations = [{
  value: "Assistant for",label: "Assistant"
},{
  value: "Student for",label: "Student"
},{
  value: "Teacher for",label: "Teacher"
}];

var oLength = [
  "Days","Weeks","Months","Years"
];

$(function() {
  function split(val) {
    return val.split(/\sfor/);
  }

  function extractLast(term) {
    return split(term).pop();
  }

  $("#occupation").on("keydown",function(event) {
    if (event.keyCode === $.ui.keyCode.TAB &&
      $(this).autocomplete("instance").menu.active) {
      event.preventDefault();
    }
  }).autocomplete({
    minLength: 0,source: function(request,response) {
      var term = extractLast(request.term);
      var results = [];
      if (request.term.indexOf("for") > 0) {
        var regE = /^(.*)\sfor\s(\d)\s(.*)$/
        if (regE.test(request.term)) {
          return false;
        }
        if (parseint(term) > 0) {
          $.each(oLength,function(k,v) {
            results.push(term + " " + v);
          });
        }
      } else {
        results = $.ui.autocomplete.filter(
          occupations,request.term);
      }
      response(results);
    },focus: function() {
      // PRevent value inserted on focus
      return false;
    },ui) {
      var terms = split(this.value);
      terms[0] = terms[0] + " for";
      // remove the current input
      terms.pop();
      // add the selected item
      terms.push(ui.item.value);
      // add placeholder to get the comma-and-space at the end
      terms.push("");
      this.value = terms.join("");
      return false;
    }
  });
});

<link rel="stylesheet" href="//code.jquery.COM/ui/1.12.1/themes/base/jquery-ui.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>

<input type="text" id="occupation" />