Here's a statistical guess:
"Elders disfellowship 50,000 to 60,000 Witnesses around the world each year, Brown said." (http://www.religionnewsblog.com/7784/spiritual-shunning). That article was in 2002, in which year there were 6,000,000 publishers. That makes about 1% per year when comparing the number disfellowshipped to the number of publishers. This jives with the stats at http://www.jwfacts.com/watchtower/statistics.php.
Applying 1% per year from 1961 to 2009 yields approximately 1,600,000 disfellowshippings.. Assuming that all those disfellowshipped in the past 50 years are still alive today, that means that ~ 1,670,000 / (7,000,000+1,670,000), 19% of the people, who have been or are JWs, have been or are disfellowshipped. The percentage number stays about the same even when applying a death rate of 0.85% per year to DF'd ones.
Numbers (publisher statistics taken from: http://www.jwfacts.com/images/publisher-table.jpg)
Year Publishers Est DF'd
1961 884,587 8,846
1962 920,920 9,209
1963 956,648 9,566
1964 1,001,870 10,019
1965 1,034,268 10,343
1966 1,058,675 10,587
1967 1,094,280 10,943
1968 1,155,826 11,558
1969 1,256,784 12,568
1970 1,384,782 13,848
1971 1,510,245 15,102
1972 1,596,442 15,964
1973 1,656,673 16,567
1974 1,880,773 18,808
1975 2,062,449 20,624
1976 2,138,537 21,385
1977 2,117,194 21,172
1978 2,086,698 20,867
1979 2,097,070 20,971
1980 2,175,403 21,754
1981 2,247,486 22,475
1982 2,342,634 23,426
1983 2,501,722 25,017
1984 2,680,274 26,803
1985 2,865,183 28,652
1986 3,063,289 30,633
1987 3,237,751 32,378
1988 3,430,926 34,309
1989 3,624,773 36,248
1990 3,846,311 38,463
1991 4,071,954 40,720
1992 4,289,737 42,897
1993 4,483,900 44,839
1994 4,695,111 46,951
1995 4,950,344 49,503
1996 5,167,258 51,673
1997 5,353,078 53,531
1998 5,544,059 55,441
1999 5,653,987 56,540
2000 5,783,003 57,830
2001 5,881,776 58,818
2002 6,048,600 60,486
2003 6,184,046 61,840
2004 6,308,341 63,083
2005 6,390,016 63,900
2006 6,491,775 64,918
2007 6,691,790 66,918
2008 6,829,455 68,295
2009 7,046,419 70,464
TOTAL 1,677,751