I was going to put this as a comment, but maybe it belongs as an answer:

For what it's worth, ArduinoJSON seems to be able to deserialize directly from a stream, i.e. Serial, which may obviate the question as asked. Presumably it knows where to stop deserializing, because it knows when it has received an entire JSON object; being a proper JSON parser, it can probably work this out better than you can without, you know, writing a JSON parser.

I was going to put this as a comment, but maybe it belongs as an answer:

For what it's worth, ArduinoJSON seems to be able to deserialize directly from a stream, i.e. Serial, which may obviate the question as asked. Presumably it knows where to stop deserializing, because it knows when it has received an entire JSON object; being a proper JSON parser, it can probably work this out better than you can without, you know, writing a JSON parser.

The following was just thrown together, now the author is here you should probably just follow their direction, but here's the code anyway:

#include <ArduinoJson.h>


void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.setTimeout(30000);
}


void print_transmission_counter( {
    static unsigned long received_object_counter;
    Serial.println(F("\n\n\n\n"));
    for (int i = 0; i < 30; ++i) {
        Serial.write('-');
    }
    Serial.write('[');
    Serial.print(received_object_counter++);
    Serial.println(']');
}


void loop() {
    static StaticJsonDocument<256> json_doc;
    static bool led_state;

    print_transmission_counter();

    const auto deser_err = deserializeJson(json_doc, Serial);
    if (deser_err) {
        Serial.print(F("Failed to deserialize, reason: \""));
        Serial.print(deser_err.c_str());
        Serial.println('"');
    } else  {
        Serial.print(F("Recevied valid json document with "));
        Serial.print(json_doc.size());
        Serial.println(F(" elements."));
        Serial.println(F("Pretty printed back at you:"));
        serializeJsonPretty(json_doc, Serial);
        Serial.println();
    }


    // Just give some visual indication that the loop is progressing
    led_state = !led_state;
    digitalWrite(LED_BUILTIN, led_state);
}


//  vim:sw=2:ts=2:et:nowrap:ft=cpp: