I was given doxycycline 100 mg for 7 days. i have completed my 7 days, how long before i need to retest again for chlamydia infection?

Any time now.

There has never been a proved failure of 7 days of doxycycline to cure chlamydia as long as the drug was taken as directed, missing few if any doses. Therefore retesting is rarely needed. If you do it, you must wait until at least 2 weeks after the last dose of doxycycline: before that, chlamydial DNA can persist (even though the infection was cured) and give a false positive test result.