As far as I know they never go on your record.
If it wasn't an official criminal arrest and charge, it will not appear on your permanent criminal record. Typically, these types of "warnings" are only kept on file by your local law enforcement...
One year. Warnings are waived tickets and if you receive another ticket in the warning period the original warning also become a ticket as well.
If you are referring to warning tickets, they will appear on the record but do not count as chargeable offenses nor do they gather points.
As long as the employer feels it needs to, unless it negotiated an individual or union contract to the contrary.
It will stay on your dmv record forever, it will affect your insurance rates for approx 5 years.