It is pot luck - if you stop early and then it comes back, you will never know whether the extra year would have made a difference. Some oncos only recommend 6 or 9 months; some prefer 2 or 3 years; some men have brachytherapy/external RT without any HT at all!
You need to discuss it with your onco or, if you have one, your clinical nurse specialist. If you are determined to stop, they can’t stop you but they will hopefully give you clear reasons / data for continuing. Some men would be impatient to stop at 2 years because that is the only way of finding out whether the treatment was successful. In the end, if your cancer did come back later, you would simply go back on the hormones.