How long for an inflamed urethra to heal? std test are negative and dna testing for bacteria was negative. 2 biopsies taken from urethra during cystoscopy showed inflamed urethra?

This is a difficult question as the cause of your urethritis is unknown. If STD & bacterial infections are ruled out it may be a viral infection due to herpes simplex or it could be an autoimmune urethritis. You could obtain a second opinion at a department of urology at a university hospital near you.

Normally 7-10 days once the cause is found and treated