Is it true that if you take dayquil for symptom management if you have a cold (pn drip/sore throat/cough) that the cold may take longer to go away?

Not necessarily : I would treat symptoms if improves the patient feeling better.Sometimes if you suppress a cough you may not clear secretions as much but if does significantly make you feel better and causes no other harm such as increasing blood pressure then I would treat it.It will likely not increase the duration of the illness that significantly.