How quickly can an aneurysm grow?

Varies. Ascending usually less than 1 cm per year but sudden expansion is a true danger. Descending similar. Abdominal perhaps a bit slower, but any pain, leakage, rapid change is a bad sign.
Depends on aneurysm. Aneurysms of the abdominal aorta are thought to grow between 4 mm and 10 mm per year based on numerous natural history studies. Any faster than this and it can be a cause for concern and an indication for surgery. Aneurysms of the thoracic aorta grow somewhat slower than the abdominal aorta, but the expected rate is similar.