I guess the question is what duration would make it usable for you. There are different approaches to fluid sim, Blenders implementation is computationally intensive, but gives good results. Also, as Richard points out, make sure you have an OpenMP build (i.e all cores will be used).
When I see how much faster Blender and Cycles rendering is when I use my GTX 570 CUDA card, it makes me wonder why baking fluids can’t also use the GPU/CUDA?
You can’t just snap your fingers and have existing code work on the GPU. Most likely you’d have to rewrite it from scratch, with all the limitations of GPUs in mind. Also, some algorithms may actually perform worse on GPUs. Fluid Sim is likely one of these problems where not being able to fit the entire workload on the GPU memory will make it either “not work” or “work too slowly”. Blenders Fluid Sim also happens to be quite memory intensive, so you’d be quite limited.