Weakness and sleepiness due to hypokalemia

My father is 74 year old till healthy active individual but has COPD (under regular medication). During very hot summer days (two weeks earlier), he developed severe weakness and sleepiness. After blood test we found there is a hyponatremia (less Na+) and hypokalemia (less K+) and his chest xray shows there are mild pneumonia infection. He was given IV Na/K replacement for three days (as well as antibiotic). We saw development in his overall health condition. Afterwards, he has been given oral K+ replacement. No doubt he is better than previous. But still lacks the energy and easily getting fatigued. I want to know whether this is common in under-treatment hypokalemia patients and how long does it take to normalize or we should worry something more serious ? Thanks in advance.