It depends a lot on the hash functions. Lots of hashes are believed to be difficult to parallelize on GPUs and memory hard hash functions have different scaling properties. But even then you need to assume that an adversary has lots of computing power and a decent amount of time. These can all be estimated then you give yourself a wide margin.
Keep in mind they will be using GPUs with lots of ram and cores so that 1s will be more like hundreds or thousands a second.
It depends a lot on the hash functions. Lots of hashes are believed to be difficult to parallelize on GPUs and memory hard hash functions have different scaling properties. But even then you need to assume that an adversary has lots of computing power and a decent amount of time. These can all be estimated then you give yourself a wide margin.