Average weight for 5'8

Depending on age and build, an acceptable average range will be from 145 lbs to 170 lbs.

That being said, there are many factors to take into consideration.

Body fat percentage and BMI are better indicators of overall fitness and health than weight by itself. Circumference measurements of different muscle groups and the neck and abdomen are also significant in dictating your proportions, above and beyond weight.

If you have a smaller build and you're a teenager (whether smaller build because you haven't really grown up yet or just because that's your destiny) 130 lbs isn't out of the question for a healthy weight either.

If you're in your fifties, of medium build, and have done a significant amount of weight training (and probably not asking this question), you could easily be 180 lbs and still be pretty healthy. Likewise, if you're in the prime of life and do a good deal of Weightlifting, 190 is even a possibility, but for that you would have to be pretty densely muscled to not be significantly overweight.

All averages taken into consideration, at 5'8" 175 is just a bit higher than you want to go without a real fitness reason, and 140 is just a bit smaller than you want to be unless you're very slight of build or very young.