Both the analysis time and the solution accuracy depend on the voxel count and the number of iterations required to obtain a converged solution. You can change the default setting using the Accuracy slider in the
(Simulation workspace > Setup tab > Manage panel > Settings) dialog.
A Fast solve is typically completed in a few to several dozen minutes, depending on the complexity of the model; an Accurate solve can take several hours. By default, the Accuracy slider is set midway between fast and accurate.

The Accuracy slider defines the:
Solution convergence is based on the maximum temperature.