http://www.lords.org/laws-and-spirit...-on,39,AR.html
Law 13 (The follow-on)
1. Lead on first innings
(a) In a two innings match of 5 days or more, the side which bats first and leads by at least 200 runs shall have the option of requiring the other side to follow their innings.
(b) The same option shall be available in two innings matches of shorter duration with the minimum leads as follows.
(i) 150 runs in a match of 3 or 4 days;
(ii) 100 runs in a 2-day match;
(iii) 75 runs in a 1-day match.