Enable button on content change in Teleric

I have this Telerik function which works on mouse out.

function MouseOut(sender, eventArgs)
    {
       var btn = $find("<%=btnSave.ClientID%>");
       btn.removeAttribute("disabled");

    }

But what I'm looking for is to enable/disable the radbutton once there is a character or more in a RadTextBox.

here is my code:

<telerik:RadTextBox ID="summary" runat="server" Rows="3" MaxLength="2000" TextMode="MultiLine" Width="100" ToolTip="My Summary"><ClientEvents OnMouseOut="MouseOut" /></telerik:RadTextBox>

Any help would be appreciated.