Uva/uvb both increase it.
I use a agromax 10,000k+uva bulb in my t5.
Try switching back to metal halide or what ever you flower with put blue spectrum bulb(s) for the last week or two of flower.
I was using the agromax pure uv bulb. I used it full time on one run and was fine. Other people had problems with burning. So somewhere between a few minutes an hour to full time depending on strain. Broad leaf plants burning the worse and easier they burn.
People report good luck with reptile cfl bulbs.