Data >> Anecdotes [1]
60% of domestic flights have wifi on them. My apologies if you are flying outside the US or using regional carriers 98% of the time (which might not be included here).
1: http://www.cnbc.com/2015/06/26/for-onboard-wi-fi-not-all-air...
(this was google result number one for 'percent of flights with wifi')