If you can build more than 150 per turn its good enough i guess. If i stay in the back i have 150 max. If i go for the run in front i try to have more than that. Like 250 buildings per turn max. Really depends on how much land you have and how many big grabs are there. If you only expand and cant attack anyone then its useless to have more than 150. Also on what strategy you use. Banker and resourcer need less because they can Cash/Research turns and industrialist needs more. With indy i usually end up with 200 buildings per turn at the end of set.