Whether you are an expat family considering a new local language for your children, or a family dealing with multiple languages, the concern about amount of language is the same.
How much of a language, over what time frame, is enough for bilingualism to be successful? Or at the least, how much is enough to make it worthwhile for the parents and the children? There are three factors you need to consider when making the decision about what is “enough”.
The first is the age of the child. Generally speaking, the younger the child, the less you need to worry about “enough”. If you are considering putting your English-speaking 1-year old in a Dutch creche three times a week, for one year, that’s fine. They probably won’t gain enough Dutch to go on to be a Dutch speaker, but it won’t harm them, or their development in English, in any way (linguistically). They may be lucky and have some knock-on effects later in life in terms of other language learning, but they may not.
If you have a 7-year old and are considering school in a new language, “enough” is a much more serious issue. A school-aged child has not only language learning to do, but also content learning to do. During the language learning period (which for older children can be 3-7 years to full cognitive fluency), the child is necessarily either losing out on content learning, or having to work really, really hard to catch up at home. The social aspects of being an older child learning a language in school are also more tricky – some kids are fine with it and some kids really struggle. So, if you have older children, the point at which an immersion experience becomes “worth it” is when there is enough time to master the language, and a long-term prospect to keep it up, and willingness on the part of the child (whenever possible).
If you are a multilingual family and want your children to master more than two languages, you need to plan as much as possible for balanced input. A common number from research is that children need a minimum of 20% total input in each language. This means about 2.5 hours a day of quality input in each language you want them to be able to use. In my experience, this number is on the low side – I find that many children who get only 20% in a language are reluctant to use the language, although they may understand it well.
The second factor to consider is your language goals. If you want your child to be a fluent speaker of a language and be able to read and write, then “enough” is going to be a serious commitment. Most families find that they can manage enough input in two languages to achieve this level in two languages, but the more languages you include the harder it becomes to find the time (and energy!) to provide adequate, good quality input. When looking at your family situation then, you plan for the amount of input that will help your children reach your language goals. If you have a minority language spoken at home and want your children to be able to speak and understand it, you may be able to get away with a couple of hours every day. If you speak a minority language at home but would like your children to be able to go home to your country to university some day, the time and effort needed to develop the language to that level will be much greater (for the parents and the children).
The third factor you need to consider is the individual child. There is a common myth that all children are little sponges and can soak up many languages. To a certain extent, it is true that young children seem to learn language more easily than adults. This doesn’t mean that it isn’t really hard work, and it doesn’t mean that it comes as easily to all children. If your child is very young and used to being communication-challenged, it may not be as hard for them to be put in an environment where they need to learn to communicate in another language. If your child is preschool aged and very shy and has difficulty adapting to new situations, the pay-off for a couple of years in a different language preschool may not be worth the price for the child. This means that each child’s situation must be considered as a part of the decision, and the outcomes may not be the same for all children in a family.
So the short answer is that there is no clear answer to what is “enough”. You must look at the language opportunities, age of children, length of exposure, amount of exposure, language goals and last but not least, the child, to find the answer to your question.