Our brace commands are to be continued until the aircraft comes to a complete stop.
Interestingly, despite the studies following the injuries sustained during the Kegworth B737 crash in the UK, there is not a standard, recommended brace position world-wide.
Unpremeditated brace commands also vary. Ours used to be "bend down, grab your ankles." After the changes to the standard brace position in the UK, following Kegworth, I suggested these were changed to "bend down, protect your head" which actually is much more effective and has been adopted by my company.