Language teachers are told that they should make sure they use the target language in class 90% of the time or more. I have no idea who came up with this number, but there is no solid research I know of that supports it. There is no research I know of that says the percentage of first language use should be 100% of the time, 80% of the time or 50% of time.
What research does say is that we need to supply a lot of compelling comprehensible input, and that using the first language is one way of making second language input more comprehensible. This can sometimes be a single word, or a short explanation.
The research also says that some uses of the first language are not helpful. They include translating everything we say into the first language, or spending a great deal of time talking about complex grammatical rules.
Stating a percentage requirement of 90% (or any percentage) says nothing about HOW the first language is used. A teacher could easily meet the 90% requirement simply by speaking the first language all period, without regard as to whether the input is comprehensible.
If we simply focus first language use on making second language input more comprehensible, e.g. an occasional translation of a problematic word or a brief explanation of grammar, I predict that use of the second language will be 90% or more. But if our goal is only to use the second language 90% of the time, there is no guarantee that it will be used in the service of providing comprehensible input.