How long it takes to be an Emergency Nurse?

A lot depends on where you are. Most places have a nursing shortage and because of that some ERs are hiring new grads right out of nursing school. Now nursing school can take as little as 2 years, for your basic RN program, or 4 years for a BSN. Good luck on becoming one we need more nurses, maybe we'll work together someday.

2 years if you go for an associates degree or 4 years if you go for a bachelors. You only need to be an RN to work in the emergency room. Some hospitals might still use LPN's in the emergency room but not sure of any that still do.

It depends on what country you are asking about.

If in Australia and not yet an RN:
Bachelor of Nursing > RN Grad Program (find one that offers an ED rotation) > Secure position in ED > start Grad Cert or Dip in Emergency or Critical Care (with option for Emergency Major) Nursing. All up to be a qualified ED RN 5 years at least. You can work in many EDs as an RN without Post Grad Qualifications, but you will be unlikely to climb the career ladder very far.
If you wish to qualify as an EN or RN Div 2 depending on your state, you may be able to secure one of the few ED EN positions. TO do this you will need:
EN student > experience > Extended Practice qualification > secure position in ED. Minimum 3 years, and no options for career advancement, no option to work with the very sick / critically ill.
Go to the websites of your local universities for more information. Search both Bachelor of Nursing and Grad Cert or Dip of Masters of Emergency Nursing.
Also, if at school, try and secure a work-experience place with a hospital that will place you in an ED.