Assuming $5.5B was the capital cost to build the park, and guest paying ~$79 per ticket at most, and an expected 10 million visitors a year, it would take at least 4.7 years to recover the cost of building the park. And that doesn't even include the operating costs, inflation, etc. So I'd say 10-20 years is a fair estimate. Building a park isn't for a quick buck - it's a long-term investment, and a risky one at that.
Wanda may want to build 15-20 parks, but when they do that, they aren't competing with Disney any more - they are competing with each other.
Also, if the Chinese government has an investment in the park, presumably they'd do a better job at protecting the intellectual property than they do for companies that aren't directly involved with the Chinese economy. Didn't Disney get another park to stop using their character knock-offs while negotiating for Shanghai?