I used KeyUp because TextChanged only fires when you LEAVE the control.
<dxe:ASPxMemo ID="txtInfo" runat="server" Width="100%" Height="76px">
var ele = s.GetText();
lblInfo.SetText(ele.length + '/4000 characters left');}" />
<dxe:ASPxLabel ID="ASPxLabel1" ClientInstanceName="lblInfo" runat="server" Text="(maximum 4000 chars)">
Hope you find it useful. Here' s a link to my forum post.