I am not able to export bulk products because time expiration. What is the suggested value to export over 20,000 products?
It depends on so many factors. A production server setting of 300 seconds is ideal, but that depends on the setup. In any case, PHP execution should not exceed 600 (seconds), in general (as mentioned in many forums).
Recommended PHP memory limit for Magento is 512M. Although you can increase it upto half of RAM installed in server.