Brief Answer:
8-16 weeks

Detailed Answer:
Dear sir, I have gone through your question and I understand your concerns.

Since the intense stress you were suffering from lasted less than three months this means that fortunately the negative effects on your immune system will be easily reversible and will not be permanent.

However, intense stress increases the blood levels of glucocorticoids and decrease circulating levels of white blood cells and killer T cells responsible for the function of the immune system.

In my opinion, it will take you from 8-16 weeks to recover completely... To fasten your recovery and in addition to meditation, I would recommend that you increase your daily intake of fresh fruits and vegetables particularly cauliflower, cabbages, and broccoli as they tend to boost the immune system.

Also, you will need to follow a daily exercising program .. Ideally you should include 30 minutes of simple exercise as walking and 5 to 10 minutes of passive stretching daily...

Also, do not forget daily deep breathing and relaxation exercises. I hope I answered your question.

Let me know if I can assist you further.
Regards

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