Check your state laws
Most states require repairs to be done within "thirty days or a reasonable amount of time" of written notice. Hot water heaters and furnaces (and possibly a couple other things) have to be fixed sooner. The term "reasonable time" usually applies to damages made by "Acts of God" (tornados, floods, etc..) where there are extreme damages that cold not possible be repaired in thirty days or less.
Check your state laws just to be sure. Chances are "reasonable amount of time" is in the lease just to keep people who don't know their rights from taking action against him when he just wants to be lazy.