Not a simple answer.
Starting point - the mesh must have sufficient resolution to get the flow field about right. Ie the bulk flow is fine and you capture all of the main features.
Refinement - will use gradient adaption (for example) to refine in regions where it's needed.
For optimising speed, you trade off remeshing time against having a larger cell count all of the time. You also trade off dynamic repartitioning against solve time. My general approach is to use enough cells to get the job done, but also to not stress the hardware I have available. Mesh refinement may well be used, but depends on the physics and purpose of the simulation.
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