When you file for bankruptcy you as a debtor you get what is called a period of 'exculisvity' where you can file you plan of reorganization. After that the creditors can file one for you. This is usually an 18 month period but can be extended by a Bankruptcy Court judge for various reasons. (see United)
I believe the new laws place a cap on exclusivity of 18 months with no exceptions.
Most Ch 11's last between 18-24 months. So considering that United is the largest Ch 11 in history it doesn't surprise me that their case has taken so long. There are a lot of issues to work out with many different parties.
As a side note to those who complain about them being in CH
11 so long. It is better to be in for a while (see Contitnental almost 5 years in early 90's)and get it right than go through the process too quickly to land back in Ch 11 soon after. (see US Airways 7 months in Bankruptcy)
Bankruptcy in this country has largely worked. It rescues companies worth saving and weeds out bad ones. (most of the time)
I do bankruptcy research for a living so feel free to ask any questions and I will do my best to answer.