I work for a major company that is changing its name. How long do I have to wait to use that name as my own company? How do I find out if the company I am working for is going to keep the registration on the name so that no one else will use it?

I would say if you work for that major company and want to keep working for them, you never use their old name. If they have a trade mark on it, they will keep ownership for it, for years as it has to do with advertising and ownership.

Now as many do, you may want to look at usng a variation of the name which is common when trying to legally "use" another companies name.