I did 301 redirect and move contents and homepage permanently from a.com to b.com. How long does it takes for b.com to ranked as same as a.com were ranked? It's about 3-4 days now and ranking not come back yet.
Be prepared to wait several months. I recently moved a 50-60 page subdomain into its own independent domain. It took almost a month for Google to update their index, and 3 months for the rankings to begin to recover. Yahoo! still shows the old subdomain for some searches - now almost 4 months later.