To qualify as a dual language program, all schools, regardless of population, must use the partner language (the language other than English) for AT LEAST 50% of the instructional day. See definition of Foreign Language in Elementary School (FLES) where instruction in the partner language does not happen for at least 50% of instruction time.
There are two basic models:
Full Immersion (90/10): In two-way and developmental bilingual programs, the partner language is used most or all of the day in the primary grades (80-90%). Foreign language (one-way) immersion programs that implement the full immersion program use the partner language for 100% of subject matter instruction, and in some cases, also offer specialist classes in the partner language. In all cases, the partner language and English are used equally in the later grades.
Partial Immersion (50/50): The partner language and English are used equally throughout the program.
Example of a 90/10 program
(National Dual Language Consortium http://www.dual-language.org)