Persons sentenced to a custodial sentence for one or more of the offences mentioned in articles 421-1 to 421-6 of the Penal Code, excluding those defined in articles 421-2-5 to 421-2-5-2 of the same code, may not benefit from the reductions in sentence mentioned in Article 721 of this code only up to a maximum of three months per year of imprisonment and seven days per month for a period of imprisonment of less than one year.