...Edited to include draws (for simplicity, count as half a win, ignoring first innings points):
Armchair expert wrote:Redbacks wins/draws
2018-19 0W 4D* (8 games) => 25%
2017-18 2W 2D => 30%
2016-17 5W 0D => 50%
2015-16 5W 0D => 50%
2014-15 2W 2D => 30%
2013-14 3W 5D => 55%
2012-13 4W 1D => 45%
2011-12 0W 4D => 20%
2010-11 1W 1D => 15%
2009-10 2W 3D => 35%
2008-09 4W 3D => 55%
2007-08 2W 1D => 25%
2006-07 1W 3D => 25%
2005-06 4W 3D => 55%
2004-05 3W 0D => 30%
2003-04 2W 2D => 30%
2002-03 4W 1D => 45%
2001-02 3W 2D => 40%
2000-01 2W 2D => 30%
1999-00 3W 2D => 40%
1998-99 2W 5D => 45%
1997-98 1W 5D => 35%
1996-97 2W 1D => 25%
1995-96 5W 2D => 60% WINNERS
So in the past 23 seasons since winning the Shield (including the current season), there have been 3 seasons with a greater than 50% record.
When comparing Win/Loss ratios in AFL/SANFL for example, normally a greater than 50% record will get you into finals (although there are obviously exceptions year-by-year), so their performance is roughly equivalent to a football team making the finals 3 times in 23 years (or if you include exactly 50% as enough, then 5 times in 23 years).
An alternative comparison is considering when they have finished in the top half (3) of the competition, which has also only occurred 5 out of the last 23 seasons.
And comparing the finals and win/loss ratios of some of the poorer performing AFL clubs over a long period of time:
Fremantle - made finals 7 times in last 24 seasons (> 50% in those 7 seasons, exactly 50% in 2 other seasons where they didn't make finals)
Carlton - made finals 4 times in last 17 seasons (> 50% in 2 seasons, exactly 50% in 3 other seasons)
Richmond (prior to 2013) - made finals 2 times in 30 seasons (> 50% in 5 seasons, exactly 50% in 3 other seasons)