Final answer:
A business is deemed to have adopted a permissible accounting method after two years of use, while an impermissible accounting method is deemed to have been adopted after three consecutive years of use.
Step-by-step explanation:
A business is deemed to have adopted a permissible accounting method if it has used the method for two years. It is deemed to have adopted an impermissible accounting method if it has used the method for three consecutive years.