.Com's and .Net's usually don't drop for +/- 80 Days after the Expiration Date - So in theory you would want to look for domains expiring approximately 80+ days ago.
BUT - If the domain has any real traffic or resell worth , Don't expect it to make it to Dropped Status. Many people are doing backorders now a days - and many are testing for traffic to see if it's worth the registration fees. If you see a domain picked up by a company or individual that you still want , It's best to keep an eye on it every 4 or 5 days after the drop date as well because if it doesn't have enough traffic to keep the catchers interest , they many times let it drop again after several days.