For well-known national or international speakers, count on 3-6 months in advance. In December, June or September you're closer to 6 months, because diaries are full then. Less well-known speakers and moderators can sometimes be booked 4-6 weeks in advance too.
The rule of thumb: the bigger the name and the more popular the period, the earlier you book. Always ask for an option so you reserve the date while you're still preparing the brief.
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