When is it to prune this? Mine is getting way too large!
When it gets way too large. ;o)
Anytime after roots are well established and you have a sharp pair of pruners in your hand.
Watch out for the Mexican Petunia ruellia because they can become very invasive. They are pretty, but once established they can go like wildfire! I put one cutting in the ground about 5 years ago, and I'm still pulling them up to this day.
i put mine in a container because i read of the invasive part. it works great!! looks beautiful!! :'))
I don't think you can kill Mexican petunia by cutting.
It's a robust perennial that also self-seeds.
If you find that you've got too many seedlings, they're very easy to pull, as the root systems are very small & short, & you can also offer them on freecycle or craigslist.
Best luck, & enjoy your garden!