When I'm on track (which has been a few months...), I usually do something like this:
Sunday - legs and shoulders
Tuesday - upper back and whatever sounds "fun" at the time, like flipping tires, etc.
Thursday - lower back and chest
Then, I may do some light upper back work again a day or two later, I might just play around with some strongman event training, or I might just take it easy and only train 3x that week.
There's no "right" way to do it for everyone, some people can train often and recover quickly, others need more time off, it all depends on factors such as genetics, nutrition, how you're training, how physically demanding your work is, etc. that will affect how is best to train for you.
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous