The economic damage the frigate can cause is always lower than its costs. There are some rare cases on setons wherenan early aon frigate can kill 4-5 rock mexes (2 island, one on rocks beach, the obvious forward one and one core mex) but even in those scenarios its hardly more than a nuisance.
I think i have alot of experience when it comes to timing pushes on navy and the key concept is, as thomas explained, build power denial. Ultimately most of my navy wins happen because of build power denial. You can scale up your bp, especially the one needed for a timely T2 transition, keep denying enemy bp with suicide frigates raids or plain water entry denial and then finish the game with cruiser tml or bship. Its of course vulnerable to torps, hover or any other mid intervention like drops, and stuff, but a satisfying way to show an eco whore its place. Think about frigates more as a tool of bp denial in the early stages of navy, and you will learn to use it as an efficient attack.
