The NPC server will either start automatically (which should take about 10 minutes max I guess), or it does not start automatically due to some problem (or keeps crashing). In the latter case, an ETA cannot be given, as it depends on devs with server access to examine the problem and solve it. This may take anywhere from a few minutes to several days, but we only know after the problem is solved
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Server status itself can easily be found out by attacking an NPC. If rats are impervious to attack, the server is down. If this is sometime after a server crash, the npcclient probably has a problem that is not quickly solved.