I talked with an immigration lawyer last year. She told me that the period of free re-entry was only within 6 months of the visa issuance. I did check and found a list of varying periods for different countries on some website (I can't find it now.)
However, I just found this paragraph on www.uscis.gov:
"An E-2 nonimmigrant who travels abroad may generally be granted an automatic two-year period of readmission when returning to the United States. It is generally not necessary to file a new Form I-129 with USCIS in this situation."
So I guess one may want to re-check to be very sure if one wants to get an E-2. I'm going to apply for the Entrepreneur Parole since it fits my situation much better.