In 2013 I built a comprehensive (or it felt like that at least) set of wooden planes during the winter. They worked wonderfully well until the humidity shot up during the rainy season. This was not a surprise as I planned to only use them during the dry season, which is most of the year anyway here in Windhoek. I knew about this tendency of the wooden planes to go out of flat with the change in seasons from all the reading I did before embarking on the building phase. One author recommended that one should actually build a winter set and a summer set, then you never have to re-flatten them.
We had a fairly wet rainy season this year and the humidity in my shop continued to fluctuate between 50-75% until mid May. I kept checking my wooden planes to see when they would return to their flat state. One week ago I started thinking that they might never return to this state without encouragement. I started developing theories on why that might be, ranging from poor workmanship to wrong grain orientation to flaws of the laminated construction/design and many more.
Finally one evening last week I decided to coerce them back into working order using 3M adhesive backed sandpaper on glass. To my surprise when I checked the soles on the flat glass prior to the planned coercion, they were spot on back to flat. It thus took about a month from the time the ambient humidity got back to around 30% (which was the humidity at which they were built and flattened) for them to return to their flat state. While I was at it I decided to do a light sanding anyway.
All the planes were exactly the same, but these pictures of the jointer illustrates it best. In the pictures below you can see how the sole looked like after a few light passes over the sandpaper. The toe, the area around the mouth and the heel were all in the same plane. That is exactly what you want for a well functioning plane. I continued to sand it all into the same plane, but it only took a few more very light passes. I can therefore confirm that these planes return to their flat state once back at the ambient humidity it was last flattened at. In this case it took about 4 weeks.