Depending on if it's a colony or a homeworld is the question. Outposts you can take over with about 5 destroyers, you'll lose some but you'll take it over. I know of someone who's currently been locked in a battle with a guys home world for pretty close to 18+ hours now using around probably 50 battle cruisers and 20 destroyers, granted he also has about 6 laser array orbitals as well. He used that same army and took an expansion colony out in a few hours.