I have very slight swelling of one tonsil with sharp pain when i swallow. no other symptoms. i suspect it is viral pharyngitis, how long will it last?

Totally depends...: ...on your own immune reaction, the specific virus & whether you get a secondary bacterial infection. If it's truly a virus, expect anywhere from a 3-10 day ride, of varying severity. An acute EBV pharyngitis can last a few days, but a chronic infection in your liver & lymphocytes can brew for weeks to months. There are a gazillion viruses that can cause a sore throat, often never identified.