1 week, 6 days after and achilles rupture, a podiatrist put a cast on with toes pointed downwards. how long in general does that cast have to stay on?

6-10 weeks: What did your podiatrist tell you? Could be 6-10 weeks. Partial or complete tear? Partial tear he might transition you to a walking cast around 4-5 weeks.

Consider surgery: If you had a complete rupture of the Achilles' tendon, you may want to consider surgery which in general has a better outcome than non operative care. That being said, in nonoperative management a cast is placed with the foot and ankle in some degree of plantar flexion and then is gradually brought up to neutral week by week into a neutral position by about 6-8 weeks.

Who Knows.: Not to trivialize your question, but it really depends on just what the treating podiatrist is trying to accomplish. It would be best to ask the rational for his treatment protocol.