How long was Severus Snape a death eater?

Answer 1

It isn't known for exactly how long he was a death eater, but it wasn't for a long period of time.

Severus Snape presumably joined the death eaters straight after leaving Hogwarts in July or August 1977.

There is then some confusion as to the timeline of events and a continuity error. In Harry Potter and the Prisoner of Azkaban it was said that the Potter's were betrayed a week after they went into hiding, which would mean that Severus met Albus Dumbledore and defected to the Order in late October 1981.

However, Sirius says that they were aware of someone from the Order passing on information to the death eaters for a year which would imply that Severus told them. This is furthur supported by the letter Lily wrote to Sirius about Harry Potter's first birthday, where they are already in hiding, two months before they died, where it is implied they have been for a considerable amount of time.

The prophecy was made before Harry was born in July 1980, and Severus was the one who overheard the prophecy and told Lord Voldemort who immediately started trying to find out who this child was.

So while it is unclear exactly when Severus defected it is likely to have been in 1980, probably in October or November as the scene in the book describes there being no leaves on the trees.

This would mean Severus was a death eater for around 3 years.