Try this
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<script type="text/javascript">
var callTimeout = function () {
setTimeout(function () { alert('hi'); }, 5000);
}
</script>
</head>
<body>
<form id="frm" runat="server">
<asp:Button ID="btn" runat="server" OnClientClick="callTimeout(); return false" Text="Test" />
</form>
</body>
</html>