Speed of the stirrer should controlled to be 20 rpm for mixing the slurry in an interval of 1 min every hour. 

General answer is impossible. Optimum mixing depends upon geometry of an anaerobic digestor and on other technical parameters.

Regards   Vit

As it was mentioned in the previous answers, there is no exact answer about this question. In general mixing velocity at full scale mesophilic anaerobic digesters is about 13-15 rpm. The slurry velocity  in digesters (which generated by the mixers) should not be more than 1.15 m/s.