I'm not an expert, but I have many friends that are (mostly European and S. Americans).... The gap is huge. One easy barometer is how many Americans actually play in the Euro leagues. And how many US National Team players play in the top Euro leagues. I may be incorrect, but only one of our National Team players (Goalie) has what it takes. One could also argue that goalies are not typical soccer players, so his ability is not a great measuring stick compared to non-goalies.
How long will it take... Every WC seems to bring us a little closer, but still a long ways to go. Soccer is the most popular sports in 90+% of the countries. That will not happen in the US for a long, long, time. While American Football took a hit with the concussion situation, it still is the strongest sport. Add in basketball and baseball and the climb for soccer is pretty steep and will be. Many factors however are helping with popularity. WC and Olympic play is driving up fans. Immigration to the US is driving up fans. How long will it take for soccer to overcome, won't happen in my lifetime (I'm 43). As a huge fan, I hope it continues to makes strides and the US becomes more competitive.