Well I found this:
http://www.nflpenalties.com/penalty/intentional-grounding?view=team&year=2015
It gives lots of information about intentional grounding penalties (and others, should you so choose). Here are the raw numbers of IG penalties called by year (regardless of whether those penalties were accepted):
2009 - 36 in 17,033 passing attempts (1 IG penalty for every 473 passing attempts; 1 per 185 incompletions)
2010 - 47 in 17,269 passing attempts (1 IG penalty for every 367 passing attempts; 1 per 144 incompletions)
2011 - 43 in 17,410 passing attempts (1 IG penalty for every 405 passing attempts; 1 per 162 incompletions)
2012 - 30 in 17,788 passing attempts (1 IG penalty for every 593 passing attempts; 1 per 232 incompletions)
2013 - 41 in 18,136 passing attempts (1 IG penalty for every 442 passing attempts; 1 per 172 incompletions)
2014 - 30 in 17,879 passing attempts (1 IG penalty for every 596 passing attempts; 1 per 223 incompletions)
2015 - 15 in 9,510 passing attempts (1 IG penalty for every 634 passing attempts; 1 per 231 incompletions)
On average, IG gets called once every 6-7 games in the NFL. So it just isn't called very often.
So this is interesting information, but unless we go back and watch every single pass play we can't tell whether any incompletion where IG is not called is similar to the plays Brady got called for.