The guys at Ryan Solutions looked at over 15,000 resort email campaigns sent since 2007 by nearly 100 different resorts and hotels. We found the average length in characters for each year and plotted the results on the graph below.
After starting at 32.1 characters in 2007, average subject line length has grown to a peak of 43.2 characters in 2014, an average of 41.9 characters since 2011, and 42.2 characters so far in 2015.
There is a similar correlation in open rates, see graph below.
In summary, longer subject lines (not too long) are improving open rates.