The measurement of eating a kezayit (an olive's worth) of bitter herbs, the Chazon Ish wrote (Orach Chaim end of Siman Lamed Tet) in the name of the Chasam Sofer (Shu"t Part VI Siman Tet-Zayin), that it is nine minutes. Even though he had difficulty in proving his point, and there is no clear indication for the measurement of eating a kezayit of bitter herbs, in any case, he concluded to rely on the ruling of the Chasam Sofer. Similarly, it is written in Shaday Chemed (Achilah section G), that this is also the opinion of the Gaon Baal Roshit Bikkurim. In Teshuvot Tzemech Tzedek (Siman Tet) it is written that it is like seven minutes. In the book Berachot Yisrael (Letter Tet-Nun-Alef in the name of Mahari"tz) it is written that it is slightly more than five minutes. In Hagadat Mo'adim Vezmanim, the opinion of the Gaon Rabbi Yitzhak Elchanan is brought, that it is five minutes. There are other opinions as well. And since the mitzvah of eating bitter herbs nowadays is rabbinical, it is possible to rely on the ruling of the Chasam Sofer regarding the amount of bitter herbs to be eaten.