This is a difficult question to answer, as people work at different speeds.  But it should be possible to complete a traditionally sprung, stuffed and stitched dining chair in the evening upholstery class in a term (36 hours in total).  You might be able to finish a small armchair in that time, but because there are more stuffed elements in an armchair than in a dining or side chair it might take you two terms.  If you work slowly it might even take you three!  The tip is to start small.  A drop-in seat is an ideal starter project.