it depends on the betta's personality.
i had bettas that charge non-stop at a mirror, i had bettas that charge for a while then minds his own business, i had bettas that have literally no interest in mirrors.
i would not recommend putting a mirror next to their 'house' all the time as it will stress them out.
as for me, i give my betta a 15 - 30 minute workout in front of the mirror, daily if i can. he'll look at it, flare, then move around abit, then flare again.
flaring is like exercising, it's good for your health but you'll die faster if you do it non-stop.