There is no set number, but if you just pull back and never let them know who or where you are, within a few years almost everyone will have forgotten about you.
Depending on your situation, this may or may not be easy. If you're single without family or much of a position in the cult, literally 1-2 years may be sufficient to no longer be known as a JW, grow a beard or color your hair or move away from the area. But then it also doesn't matter to give in your resignation from the cult, you've got nothing to lose.
If you have friends and family that's still in and "faithful", you'll lose them even if you don't get disfellowshipped, there is no winning there. I haven't gotten disfellowshipped myself but I'm still known as a staunch apostate, last time JWs came to my door they literally ran away (almost 4 years later)