So you edit this file

/usr/share/koji-web/scripts/hosts.chtml

change the displaying if-block

    #if $len($hosts) > 0
      #for $host in $hosts
        #if (($time.time() - $time.mktime($time.strptime($util.formatTime($host.last_update),'%Y-%m-%d %H:%M:%S'))) < (15 * 60)):
          #continue
        #end if
        <tr class="$util.rowToggle($self)">
          <td>$host.id</td>
          <td><a href="hostinfo?hostID=$host.id"><b><font color="red">$host.name</font></b></a></td>
          <td>$host.arches</td>
          <td class="$str($bool($host.enabled)).lower()">#if $host.enabled then $util.imageTag('yes') else $util.imageTag('no')#</td>
          <td class="$str($bool($host.ready)).lower()">#if $host.ready then $util.imageTag('yes') else $util.imageTag('no')#</td>
          <td>$util.formatTime($host.last_update)</td>
        </tr>
      #end for
      #for $host in $hosts
        #if (($time.time() - $time.mktime($time.strptime($util.formatTime($host.last_update),'%Y-%m-%d %H:%M:%S'))) >= (15 * 60)):
          #continue
        #end if
        <tr class="$util.rowToggle($self)">
          <td>$host.id</td>
          <td><a href="hostinfo?hostID=$host.id"><b><font color="green">$host.name</font></b></a></td>
          <td>$host.arches</td>
          <td class="$str($bool($host.enabled)).lower()">#if $host.enabled then $util.imageTag('yes') else $util.imageTag('no')#</td>
          <td class="$str($bool($host.ready)).lower()">#if $host.ready then $util.imageTag('yes') else $util.imageTag('no')#</td>
          <td>$util.formatTime($host.last_update)</td>
        </tr>
      #end for
    #else

– OR THE HARDER WAY –

edit

/usr/share/koji-web/scripts/includes/header.chtml

add this line right before the ending </head> tag

#include "includes/hosts.js"

and you create a file in the same directory named hosts.js with

<script>
	var hoststat = 0;
	
	function conmonth(monthstr)
	{
		var x, l, m;
		var monthary = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
		
		l = monthary.length;
		m = monthstr.length;
		
		for (x = 0; x < l; ++x)
		{
			if (monthary[x].substring(0, m) == monthstr)
			{
				return (x + "");
			}
		}
		
		return (0 + "");
	}
	
	function procdown(rowdlist)
	{
		var x, l = rowdlist.length, z;
		var coldlist, objclist;
		var locldate, loclutct;
		var hostdate, hostutct;
		var logiinfo = document.getElementById("loginInfo");
		var dateobjc = logiinfo.innerHTML.replace(/[\t\r\n]/g, "").match(/^.*[^ ]+ ([^ ]+) ([^ ]+) ([^ ]+) ([^ ]+) UTC.*$/);
		
		locldate = (dateobjc[3] + ":" + conmonth(dateobjc[2]) + ":" + dateobjc[1] + ":" + dateobjc[4]).split(":");
		loclutct = new Date(locldate[0], locldate[1], locldate[2], locldate[3], locldate[4], locldate[5]);
		
		for (x = 0; x < l; ++x)
		{
			coldlist = rowdlist[x].getElementsByTagName("td");
			
			if (coldlist.length < 6)
			{
				continue;
			}
			
			hostdate = coldlist[5].innerHTML.replace(/^[^0-9]*/g, "").replace(/[^0-9]*$/g, "").replace(/[^0-9]/g, ":").split(":");
			
			if (hostdate.length < 6)
			{
				continue;
			}
			
			objclist = coldlist[1].getElementsByTagName("a");
			hostutct = new Date(hostdate[0], parseInt(hostdate[1].replace(/^[^1-9]*/g, "")) - 1, hostdate[2], hostdate[3], hostdate[4], hostdate[5]);
			
			if (objclist.length > 0)
			{
				if ((hostutct.getTime() / 1000) <= ((loclutct.getTime() / 1000) - (15 * 60)))
				{
					objclist[0].innerHTML = ("<b><font color=\"red\">" + objclist[0].innerHTML + "</font></b>");
				}
				
				else
				{
					objclist[0].innerHTML = ("<b><font color=\"green\">" + objclist[0].innerHTML + "</font></b>");
				}
			}
		}
	}
	
	function redotabl()
	{
		var x, l;
		var downlist = [], rowdlist = ["row-odd", "row-even"];
		var tabllist = document.getElementsByClassName("data-list");
		var tablrowd;
		
		if (tabllist.length > 0)
		{
			l = tabllist[0].rows.length;
			
			for (x = 0; x < l; ++x)
			{
				if (tabllist[0].rows[x].innerHTML.replace(/[\t\r\n]/g, "").match(/^.*font.*color.*red.*$/))
				{
					downlist.unshift([x, tabllist[0].rows[x].innerHTML]);
				}
			}
			
			l = downlist.length;
			
			for (x = 0; x < l; ++x)
			{
				tabllist[0].deleteRow(downlist[x][0]);
			}
			
			for (x = 0; x < l; ++x)
			{
				tablrowd = tabllist[0].insertRow(3);
				tablrowd.innerHTML = downlist[x][1];
			}
			
			l = tabllist[0].rows.length;
			
			for (x = 3; x < l; ++x)
			{
				tabllist[0].rows[x].className = rowdlist[(x - 3) % 2];
			}
		}
	}
	
	function hostloop()
	{
		var oddrlist = document.getElementsByClassName("row-odd");
		var evenlist = document.getElementsByClassName("row-even");
		
		if ((oddrlist.length > 0) || (evenlist.length > 0))
		{
			procdown(oddrlist);
			procdown(evenlist);
			redotabl();
			
			hoststat = 1;
		}
		
		if (hoststat == 0)
		{
			setTimeout("hostloop();", 0.5 * 1000);
		}
	}
	
	if (location.href.match(/^.*hosts.*$/))
	{
		hostloop();
	}
</script>