gistfile1.m

void CRWaitMinimumDurationAndExecute(NSTimeInterval start, NSTimeInterval minimumDuration, void(^block)(void)) {
    double diff = [NSDate date].timeIntervalSince1970-start;
    double delayInSeconds = MAX(0.0, minimumDuration-diff);
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        if (block) block();
    });
}

//Usage

NSTimeInterval start = [NSDate date].timeIntervalSince1970;
NSTimeInterval minimumDuration = 2.0
[[SomeAPIClient sharedInstance] getWithCompletion:(void (^)(void))completion {
    CRWaitMinimumDurationAndExecute(start, minimumDuration, ^{
        //Stuff that will happen either 2 seconds later or whenever completion ends up getting called if that is > 2 seconds
    });
}];