It depends.
Restarting a simulation requires a .RESTART file for each mesh in the simulation. PyroSim has a default setting to have FDS write these files every 300 s (Analysis -> Simulation Parameter -> Output -> Restart). So, with the default setting, the simulation has to run for > 300s for a restart to be possible. If the simulation runs for 599 s and dies due to a power failure, then it will restart at 300 s. If it runs to 601 s, then it will restart at 600 s.
If you run FDS through the PyroSim interface, then there are two options; you can Kill the job or Stop the job. If you choose the latter, then the .RESTART files are written as the job shuts down.
See section 7.3 of the FDS Users Guide.
The developers describe the restart feature as being fragile; it has become more reliable over the years, but the longer the simulation has run, the less likely you are to be able to successfully restart it.
Dave