Yes, board members should be allowed to take a leave of absence. Someone may be out on maternity leave, need some time off to care for a sick loved one, be temporarily relocated for a work project etc. If they remain on the board and don't attend it can lead to challenging group dynamics and affect your quorum. So if someone really knows that they won't be able to attend for 4-6 months or as much as a year, then have them officially take a leave of absence, by submitting a formal request in writing to the chair of the board. This request should be formally acknowledged in writing. While they are on an official leave of absence they are no longer part of your quorum or officially a trustee during that period of time. If a trustee requires a year or more off, it is probably better to have them resign from the board, and be considered for re-nomination at a later date.
Updated