If i'm taking 800mg dose of ibuprophen 3x day for tendonitis (tennis elbow), how many days should i/can i take it?

As needed: Take until pain subsides. It is safe to take for extended periods, but likely your doctor did not intend for you to be on it for life. Watch for signs of stomach irritation or upset. If this develops, then stop the med. If you take it regularly for more then a year, have annual blood tests to monitor your kidneys and liver. Just like many long term meds, these can start to affect your organs.

2-3 weeks: If your tennis elbow is acute, then nsaids may help you overcome this condition. If the condition has lasted for more than 6 weeks, then nsaids are less helpful. Consider 2-3 weeks of use max, assuming you don't have any contraindications such as stomach ulcers, kidney disease, uncontrolled blood pressure, concurrent use of blood thinners, etc.