This is recommended every 6 - 8 weeks and it allows me to thoroughly check the extensions to make sure there isn't any tangles and that the hair hasn't become matted. I also check each bond / ring to make sure they are all still intact. With the micro ring extensions I will tighten if necessary and I will also apply an extra 25 extensions to replace any you may have lost.
Some clients choose to leave extensions in without having a maintenance check on them, this is done at your own risk and I will not be held responsible for any damage caused over the time the extensions are in your hair after the 6 - 8 week period.