How much can I cut back my gardenia?

i would cut them down about6 inches

Obviously happy and thriving! You have what everyone else wants! Here is a link for you.

https://www.gardeningknowhow.com/ornamental/flowers/gardenia/when-to-prune-a-gardenia.htm

I have cut full sized azaleas(5-6+ ft high) and several other plants in that same family back 50% of their height and be very happy with the results. Wait until the blooms are off, but then cut them back. Don't wait too late because they will put out new growth that needs to harden before it gets cold.

Prune shrubs after they have finished flowering in summer to remove straggly branches and faded flowers.

See if you don't find this informative:

http://www.clemson.edu/extension/hgic/plants/landscape/shrubs/hgic1065.html

My Dad has kept a gardenia in a large pot for years. As it gets too large, he literally chops it down to only 6" tall. They produce new leaves on both green and brown wood so it always come back with a vengeance!!