You don't need to worry about boiling extra for extracts, most of that has been taken care of in the production process.
The reason Pilsner malt needs to be boiled longer is because low kilned malts contain higher levels of SMM which is a precursor to DMS. DMS is a sulfur compound that gives a cooked vegetable character to the beer, and boiling converts SMM to DMS and drives it off. Since extracts have already been boiled, you don't have to worry about it. You really only need to boil extracts for a few minutes to make sure they're sterile, search "late extract addition."