I had a hard time figuring out how to access general simulation parameters such as the maximum simulation time max-sim-time at runtime from whatever module needs it.

For modules it is rather straightforward. From any cModule object, there’s a par(const char* parname) function. Say you’re writing a module named OmniscientEntity (as I did), and it is defined in OmniscientEntity.ned as this:

simple OmniscientEntity {            	    
    parameters:
        // Used for CQI computation. Copy over from your channel.xml.
        double targetBler = default(0.001);
        double lambdaMinTh = default(0.02);
        double lambdaMaxTh = default(0.2);
        double lambdaRatioTh = default(20);
        // The simulation time at which final configuration takes place.
        double configTimepoint = default(0.005);
        // The entity will take a snapshot of the some network statistics every x seconds.
        double updateInterval = default(0.05);
        // 180kHz is LTE's bandwidth per resource block as per its specification.
        double resourceBlockBandwidth @unit(kHz) = default(180kHz);
    @display("i=block/table");
    @class(OmniscientEntity);    
}

then to access its parameter targetBler, just call par("targetBler") on its object and that’s it. You can traverse the module hierarchy using getParentModule(), or get to the top via getSystemModule().

This won’t work with general parameters set in your omnetpp.ini, though, because they’re not part of your module. I had thought that you would be able to access them via

getSystemModule()->par(...) // doesn't work

but apparently that also doesn’t work. Go figure.
After some significant tinkering, I finally found in the documentation that you can get access to the configuration via the globally available getEnvir()->getConfig(). From there you have functions that get you what you want, such as

getAsDouble(cConfigOption *option, double fallbackValue=0)

Cool, but what’s a cConfigOption? It has a big, nasty constructor:

cConfigOption(const char* name, bool isGlobal, Type type, const char* unit, const char* defaultValue, const char* description)

Learning by example is easiest, so to finally find the sim-time-limit you’re looking for, do this:

cConfigOption simTimeConfig("sim-time-limit", true, cConfigOption::Type::CFG_DOUBLE, "s", "300", "");
double maxSimTime = getEnvir()->getConfig()->getAsDouble(&simTimeConfig);

and adjust as you need for other parameters. Talk about easy.