Near as I can tell: They used the back up hyperdrive to get to Bespin which would have still taken them awhile but it does in fact mean Luke only got a few weeks of training. But Yoda specifically mentions that Luke has not had all of his training when he

Luke's training being incomplete raises another problem: when we rejoin Luke at the beginning of Return of the Jedi he seems to have made huge strides toward being a proper Jedi. He seems calmer, he's using his mind trick to get past guards, etc. 

We know he didn't go back to Yoda or talk to Obi-Wan because if he did he presumably would've addressed the issue of Vader being his father and why they lied to him. The only loophole I can see is that The Force runs strong in the Skywalker family thanks to their (ugh) high midichlorian count so maybe once he got the basics he was able to figure out the rest on his own. 

Don't even get me started on the gang's sloppy plan to rescue Han!