Usually what we don’t find in the studies is what the timeframe is for a material to be completely offgassed. (Seems strange not to study that, but it’s true). Also, offgassing times vary a lot by the conditions: temperature, humidity, and airflow mainly. Inside the cabinets, for example, there is almost no air flow so the formaldehyde gets reabsorbed.
Another variable is the type of formaldehyde and if there are formaldehyde scavengers (like ammonia) added. Urea-formaldehyde offgasses at a higher rate than phenol-formaldehyde.
One study for a high-emitting laminate floor product sample tested in small environmental chambers found that the time needed for formaldehyde emission to reduce to 50% of the initial level was 216 days, and the worst case was 371 days (depending on the conditions).
According to my math, it would take from approximately 1080 days (or around 3 years) to 2226 days (or around 6 years and 1 month) for the formaldehyde emissions to become negligible and hit close to 0, or 0 emissions.
Usually at 7 years, even the most extremely chemically sensitive can do well with high emitting Particleboard, MDF, and HDF and they would consider it offgassed. Though newer glues are an improvement and don’t offgas for as long.
But when it’s inside a closed area the residual formaldehyde can linger. It’s not necessarily an amount that would still show up on a measurement.
It’s difficult to measure the formaldehyde at these extremely low levels or to measure when it’s completely done offgassing because the wood itself naturally contains formaldehyde, and also the air indoors and outdoors everywhere contains formaldehyde so usually you might see a measurement like “not detectable against background levels”.
When someone is hypersensitive and is reacting to MDF, HDF, and particleboard that are many years old always look to mold as an underlying source as I found that has been the culprit in 100% of cases. The Gupta program is also very helpful with hyperactivity, I no longer have multiple chemical sensitivities because of that.