My longest DIY attachment was 24 days. I was on a hiking trip in a remote place sleeping in dormitories and tents, so I had no privacy to re-do it. I knew that would likely be the case. My experience agrees with Toobad; it would take a very long time for the adhesive to get to the point where the hairpiece would come off, but it emulsifies with your scalp oils and becomes like axel grease. If you wear a lace piece it starts to migrate through the base mesh and get on the hair shafts. If you wear a skin piece it starts finding its way out at the edges of the unit, including at the hairline and in your side hair. It felt a bit grungy and insecure - not a nice feeling. I was glad to get back home and rip it off. The clean-up on a piece which you have left on that long is pretty difficult too.
Noah