How long does a TDataset bookmark remain valid?

I have code like below in a project I'm working.

procedure TForm.EditBtnClick(Sender:TObject);
begin
  // Mark is form variable. It's private
  Mark = cdsMain.GetBookmark;
  // blabalbal
  .
  .   
  .
end;

procedure TForm.OkBtnClick(Sender:TObject);
var  
  mistakes: Integer;
begin
  //Validation stuff and transaction control
  //removed to not clutter the code
  If cdsMain.ChangeCount <> 0 then 
    mistakes := cdsMain.AppyUpdates(-1); 
  cdsMain.Refresh;
  try
    cdsMain.GotoBookmark(Mark);
    // Yes, I know I would have to call FreeBookmark
    // but I'm just reproducing 
  except
    cdsMain.First;
  end;
end;

Personally, I do not use bookmarks much — except to reposition a dataset where I only moved the cursor position (to create a listing, fill a string list, etc). If I Refresh, update (especially when a filter can make the record invisible), refetch (Close/Open) or any operation that modifies the data in the dataset, I don't use bookmarks. I prefer to Locate on the primary key (using a TClientDataset, of course) or requery modifying the parameters.

Until when is a bookmark valid? Until a Refresh? Until a Close/Open is done to refetch data? Where does the safe zone end?

Consider in the answer I'm using TClientDataset with a TSQLQuery (DbExpress).

Like both c0rwin and skamradt already mention: the bookmark behaviour depends on the TDataSet descendant you use.

In general, bookmarks become invalid during:

1. close/open
2. refresh (on datasets that support it)
3. data changes (sometimes only deletions)

I know 1. and 2. can invalidate your bookmarks in TClientDataSets. I am almost sure that for TClientDataSets it does not matter which underlying provider is used (TSQLQuery, TIBQuery, etc).

The only way to make sure what works and what not is testing it. Which means you are totally right in not using them: bookmarks have an intrinsic chance of being unreliable.

To be on the safe side, always call BookmarkValid before going to a bookmark.