Is it valid if a get is given to a women with a condition say.. if you move to china within 50 years of the giving of this get the get becomes null and void from day one and anyone she remarried during these 50 years makes her an eishes ish and chyiv misah and any children from this union are mamzeirim?
The Shulchan Aruch EH 143:20 when discussing that a get can be given on a condition, so long as it doesn’t specifically bind her for the rest of her life, mentions exactly your case of 50 years:
צריך שלא יתנה עליה תנאי שעומדת בו כל ימיה כגון ע"מ שלא תאכלי בשר או שלא תשתי יין לעולם או כל ימי חייכי שאם התנה כך אין זה כריתות אבל אם אמר לה כל ימי חיי או כל ימי חיי פלוני או עד חמשים שנה ה"ז גט ויש מי שכתב שאפי' הרחיב הזמן יותר מכדי חיי האדם כיון שהוא דבר פסוק הרי זה גט:
He must not require of her any conditions which apply for her entire lifetime such as: "on condition that you do not eat meat or drink wine", or "that you never drink wine", or "for all the days of your life" as if he forced her to to accept these conditions this is not a dissolution of the marriage. However, if he told her, [that the condition applied]: "all the days of my life", or "all the days of ploni's [a hypothetical third person's] life", or "for fifty years", behold - this is a [valid] divorce. There are those who hold that even if he expands [the duration of the conditions] beyond the length of a persons life, because the time is [explicitly enumerated and] decided this is a [valid] divorce
In the next Halacha, he mentions a case of not going to her fathers house, which the SA says shouldn’t be done since it’s almost impossible to rely that she will do that. However moving to china would be a legitimate condition.