How much is emergency tax? Emergency tax can be up to 50% of your wage.
How long will I be on it? Emergency tax codes are only ever supposed to be temporary.

When you leave a job, your employer should give you a P45. Your P45 shows how much you've earned and how much tax you've paid so far in the current tax year. It also gives your tax code, which your new employer will need. If you don't get a P45, your new employer will have to use an emergency tax code until they can find out what tax code you should be on.