Doubt it

I did Duolingo for 160 days to prepare for a trip to El Salvador. It helped a ton, but formal lessons would have taught me probably 10x faster. I continued until I went for about 2 years. After stopping, I still retained a lot, but the best way is more formal education/lessons and diving into the...

Not proficient but certainly learning some vocabulary

I did. I took 2 years of Spanish in high school. That had the same almost useless effect that it had on every other high school student, but it did lay some groundwork. Over 10 years later, I achieved a conversational level of Spanish using maybe 80% Duolingo. It was inefficient, bu...