There are no set rules on this. You're "inactive" after six months, alright, but because the WTS has published no formal rules about when your status as a JW terminates, technically, once you're baptized, you're a JW forever unless you're disfellowshipped or disassociated. However, in practice, what elders or the Society can get away with doing to you, or whether they even might want to, depends on any number of factors. Such factors include:
How long have you been inactive?
Are you known in the area as having been a Dub?
How prominent in the organization were you?
Do you make waves for local Dubs?
Do any Dubs view you as a threat?
How rabid are local elders to "deal" with former JWs?
Have you done anything to attract attention to yourself in Bethel?
A number of posters were inactive for more than five years, but remained in the area where they had been Dubs. Eventually the local elders decided to DF or DA them. I've been inactive for more than 20 years, but haven't been viewed as a Dub for most of that time, so even though I'm an active opposer on the Net, I've not been DF'd or DA'd. I also threatened a lawsuit for slander when I was divorced ten years ago, if they did anything. The Society is well aware of the fact that I would almost certainly win a massive lawsuit for slander if they did so today. Note that I have virtually no contact with local Dubs unless they come to the door, and I simply take their trashy literature and turn them away.