Every tire has a tread wear indicator (TWI Tread Wear Indicator). These indicators, which are small bumps of 1.6 mm, must be observed preventively. When wear reaches these indicators, it is recommended to replace the tires.

You can also consult Resolution 558/80 of CONTRAN - National Traffic Council on the link below: http://www.gov.br/.../conteudo-denatran/resolucoes-contran