i ride about 3 times a week, sometimes more. if i trail ride it's much longer. i try to change it up alot. i'll ride western, english and bareback (i'm e loser, i can't stay on at the canter bareback on my own horse!), and we trail ride, mostly in western tack.
good question as to if that's enough. i've heard for people getting in shape you are supposed to work out a day, and then rest, to let your muscles rebuild and repair, i would think in some ways it would be the same for horses?
Marianne and Twist
The wind of heaven is that which blows between a horse's ears. ~Arabian Proverb