Next time, look at your chat window. You will see a message that the player may have vioalted Chess.com's 'Fair Play' policy and may have their account limited.
Not sure how you would consider it watsted time. If you sit down to a game with a 30 minute time control, then you have basically dedicated the next hour to the game.
If they leave, just use that time to read some chess material or do some tactical puzzles.
ANd, when you are finished waiting for their clock to run down, you have the point and have learned something that will help you win your next game. 
Make lemonade baby, lemonade.
I'm not sure what policy change has affected this, but I find that whenever I'm winning a game or get someone's queen, instead do resigning or abandoning, they just disconnect. I wasn't having this problem before bit it happens often now. Since I don't want to be punished for abandoning and don't want to unfairly resign, this causes me a lot of wasted time. Is there a problem with punishing people for abandoning? I mean, is it really that important? I never mind if people abandon，or just treat abandon, disconnect, and resign as the same for points?? Anyway, hope there is some way to address ther disconnecting issue.