Sekis occur about once in every 50 games, more or less; or, in about 2% of all games. The occurence of two sekis in the same game is even more remote, about once in every 1,600 games, or in about 0.060% of all games.
This information is based on the statistics of about 17,000 games (as of January, 2012).
RobertJasiek: Looking through a few hundred Japanese pro games, the seki rate was 1:70. Among KGS 4d+, the seki rate is somewhere between 1:2 and 1:10; I have not done exact counting yet. Hence the seki rate depends heavily on playing style and thinking times.