I still do this, with some caveats. If I am combing and heat styling my hair regularly, I can only go 9 months without a trim.
In the past, my indicator that I needed a trim was if my hair ends started to tangle. While at the university, I cut the front of my hair down to about 4 inches long. I think I cut off about 2 to 3 inches. I washed my hair that following week.
Despite having freshly cut hair, my ends tangled and knotted anyway, just 7 days after being CUT, not just trimmed. For my hair, there were some other things to consider besides tangled ends as an indicator of my hair health.
Since then, I have stopped combing my hair regularly and rarely use a comb.
In the last 2 years I added Shea butter to me hair, oil my ends and protect them at night and wear my braids up in a baggie. I implemented a 'Nightly Ends Routine', which I did not have until around December 2016.
I have not trimmed in 2019. I've only cut out 1 or 2 small tangles. I plan on trimming in February 2020.
Do I have splits? Yes. Do I have knots? Yes. They are an issue and detract from the beauty of my hair style when I wear my hair heat straightened. Thus, I always get a deep trim/cut when I get my hair straightened.
The last time I got my hair heat straightened was in Sept 2014, which is about five (5), going on six (6) years ago. My ends are holding up okay. The 'true test' will be how it looks when I finally decide to get it heat straightened again.