outcrossing strain - how many rounds?

Is there a generally accepted number of rounds necessary to outcross a mutant strain? I’ve successfully outcrossed a strain x6 - is this sufficient?

6 rounds should be fine for most things. you should be aware though that the strain you are using for your outcrossing might harbor unanticipated sequence variants (see Sarin paper).

Genetics. 2010 Jun;185(2):417-30. Epub 2010 May 3.
Analysis of multiple ethyl methanesulfonate-mutagenized Caenorhabditis elegans strains by whole-genome sequencing.
Sarin S, Bertrand V, Bigelow H, Boyanov A, Doitsidou M, Poole RJ, Narula S, Hobert O.
Department of Biochemistry and Molecular Biophysics, Howard Hughes Medical Institute, Columbia University Medical Center,New York, New York 10032, USA.