Per the comments below it's what you make it and some groups skew more towards a traditional "God" understanding
my experience in general with norcal AA groups is "higher power" is used instead of "God" almost exclusively, people are very careful to not push God / religious understanding / etc
At step 3 you have to turn it over to a "higher power" which is a reflection of the understanding that it is unlikely you, yourself have the power to control alcohol and there must be something "Greater than" you - that can be "physics" that can be "the moon" and a guy who has been sober a very long time in the meeting i go to says his higher power is the "light fixture"
If I had to give you a number, 20% (with a church being 100%)?
But it really depends on the group. There non-religious modifications to the AA program. They don't say a prayer every 3 minutes. It definitely worth going to a meeting or two just to listen to their stories.
My brother goes to one in Utah where they're actually very careful to stay away from any religious aspects. They understand that not everyone in Utah is LDS and they're very careful to make it so everyone feels included and isn't turned off by any religious undertones. I went with him to support him for his very first meeting and it was surprising how strict this rule was.
