Congratulations! You should be proud of yourself! 
You have already received excellent advise, including that if you are asking, it is already likely time.
When I was teaching, I would say about 70-80% of the time riders were ready to move on, I approached them. I would suggest some other trainers in the area that I respected myself, depending on that student’s personal goals, discipline of choice, and personality (like not sending the super timid riders who needed a little extra reassurance to the local shark!). I would suggest they watch that trainer give a few lessons, and then we would talk about it at their next lesson with me. What went well? Did they have any reservations? If so, we talked through it, or I would try to find a more suitable fit for them elsewhere. Then they would take a trial lesson with their new instructor, or a few. Finally, especially for my students who had “grown up” with me, no matter their age, I would invite them for a final ride with me. I would usually ride with them, either in the ring, or sometimes we would trail ride. I think as an instructor, having a “kid” outgrow me felt like a personal win for me, too… it meant I served that student well, and got them well on their way.
So, maybe ask outright if you trainer has any personal recommendations for you when you have the conversation. And if you are especially close to your trainer, and/or have been there for a very long time, consider ending on a personal note, to celebrate how far both of you have come.