I sprained my knee two weeks ago and am on crutches. I work on my feet walking and talking to people. No seat. When should I return to work?

Depends. Mcl strain usally take six week to heal followed by six weeks of rehab a functional brace can help with standing with stasitting shoudlnt' be a problem for a knee strain.Depends on which ligament how bad of sprain lcl nver heal some that dont heal need reconstruction see an orthopedist.
Depends on type. A sprain by definition is an injury to the ligament complex. A sprain can mild, moderate or severe. The answer to your question depends on the severity of the sprain and what ligament or ligaments are involved. If swelling in minimal, you can move you knee and you can weight bear comfortably with crutches then you might be able to return to work provided that you can ice and elevate if swollen.
Lose the crutches. If it is just a sprain, you should not still be on crutches. You should have a knee sleeve or ace wrap for pressure to keep the swelling down and be walking as slow as you need to with as normal of a gait pattern as possible. Any medication you are taking for pain should help with this gait normalization. The longer you yous crutches or limp around, the longer your recovery will be.