If you are using the provided by Telerik Link Manager dialog there is no problem inserting 1000 characters in the ToolTip field. In order to enable the dialog you need to modify the respective ToolsFile.xml and ListToolsFile.xml, which are located in the \Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\188.8.131.52__1f131a624888eeed\Resources folder.
Just add the following code.
Regarding the tooltip
visibility time, unfortunately this is a browser functionality and the RadEditor can not control it.
the Telerik team