General consensus suggests that standard tDCS sessions can last anywhere between 5 to 30 minutes, with the most common professional research sessions lasting 20 or 30 minutes in duration. Clinical research has found five minutes to be the minimum stimulation duration required to produce notable effects which last beyond the conclusion of the session, and that sessions lasting 20 to 30 minutes often  produce the best results1.
1 Nitsche and Paulus, 2000, Nitsche and Paulus, 2001, Nitsche et al., 2003