Doxycycline will need to be taken for minimum of 7-10 days based on the severity. It will need to be followed up after initial treatment has been initiated, in 3-5 days time. Doxycycline is useful in URTI (Upper Respiratory Tract
Infection) but it should be suitably prescribed by the medical Practitioner and followed up, As said you will need to continue it for 5 days and see your treating doctor if you have no decrease in symptoms to change to a suitable antibiotics.
NOTE: Antibiotics are meant to be taken only on prescription of Medical Practitioner otherwise there will be emergence of resistance to the antibiotics. The action of those medicines in future for life threatening infections if any will be negligible. At present there are many antibiotics who have become resistant to infections due to improper use hence taking the emergence of newer and costly antibiotics