Technically, there is no statute of limitations. When you are disfellowshipped or disassociate yourself, they have to fill out a S-77 form. This is filled out in multiple copies. One is kept on congregation records, and another is sent back to Brooklyn in a Special Blue Envelope. These are never destroyed. Thus, once you are disfellowshipped, you are never going to revert back to being a worldly person in their eyes. You either get reinstated (which fact will be noted on the same S-77 form) or you remain disfellowshipped until you die (which fact is also recorded on the same form).
However, they are not likely to perfectly remember that. If you are out for a good number of years, chances are good that the hounders have totally forgotten about you. And, that is more true if there are a fair number of hounders that have moved out of the location and new hounders have been appointed. People will also forget about it, as will new ones that have no way of knowing. If you have been out more than 20 years, there is a chance that they will view you as a worldly person (though don't count on your family members still in to do that).
If they do forget, there is the odd chance that they will still find out. Sooner or later, the hounder-hounder is going to go through those S-77 forms on file in the congregation. And there's your card, without the indication of reinstatement. At which point they will once again view you as disfellowshipped or disassociated. As to whether a new announcement will be made or not, that depends on whether or not they see the need (I never heard a new disfellowship announcement made on the same disfellowshipment). But usually the hounders will then caution anyone that might be associating with the person that they are disfellowshipped/disassociated, and warned to cease such.
