Установка только одного таймера обратного отсчета для работы в повторяющемся цикле при нажатии

Вы можете создать виджет, который имеет логику таймера для каждого элемента. Что-то вроде

for (var breakdown in snapshot.data![index].breakdowns) {
  BreakdownCard(
    key: ValueKey(breakdown.order),
    label: breakdown.order);
}

class BreakdownCard extends StatefulWidget {
  final String label;
  const BreakdownCard({super.key, required this.label});

  @override
  State<BreakdownCard> createState() => _BreakdownCardState();
}

class _BreakdownCardState extends State<BreakdownCard> {
  int _countdownTime = 45;
  Timer _countdownTimer = Timer(const Duration(seconds: 0), () {});

  void startCountdownTimer() {
    const oneSec = Duration(seconds: 1);
    _countdownTimer = Timer.periodic(
      oneSec,
      (Timer timer) => setState(
        () {
          if (_countdownTime < 1) {
            timer.cancel();
            _countdownTime = 45; // Reset countdown timer to 45 seconds
          } else {
            _countdownTime = _countdownTime - 1;
          }
        },
      ),
    );
  }

  void _handleCountdownTap() {
    if (!_countdownTimer.isActive) {
      startCountdownTimer();
    }
  }

  @override
  Widget build(BuildContext context) {
    return Card(
      margin: EdgeInsets.zero,
      child: Padding(
        padding: const EdgeInsets.only(bottom: 8.0, right: 8.0),
        child: Row(
          crossAxisAlignment: CrossAxisAlignment.center,
          children: [
            Expanded(
              flex: 1,
              child: Text(
                widget.label,
                textAlign: TextAlign.center,
              ),
            ),
            SizedBox(
              height: 30,
              child: GestureDetector(
                onTap: _handleCountdownTap,
                child: Container(
                  decoration: BoxDecoration(
                    color: Colors.blue,
                    borderRadius: BorderRadius.circular(8.0),
                  ),
                  child: Center(
                    child: Text(
                      '$_countdownTime seconds',
                      style: const TextStyle(
                        fontSize: 15,
                        color: Colors.white,
                      ),
                    ),
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

Что-то вроде этого?

import 'package:flutter/material.dart';
import 'dart:async';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'My App',
      home: CountdownExample(),
    );
  }
}

class CountdownExample extends StatefulWidget {
  @override
  _CountdownExampleState createState() => _CountdownExampleState();
}

class _CountdownExampleState extends State<CountdownExample> {
  List<int> _countdownTimes = List.filled(3, 45);
  final List<Timer?> _countdownTimers = List.filled(3, null);

  void startCountdownTimer(int index) {
    const oneSec = const Duration(seconds: 1);
    _countdownTimers[index] = new Timer.periodic(
      oneSec,
      (Timer timer) => setState(
        () {
          if (_countdownTimes[index] < 1) {
            timer.cancel();
            _countdownTimes[index] = 45; // Reset countdown timer to 45 seconds
          } else {
            _countdownTimes[index] = _countdownTimes[index] - 1;
          }
        },
      ),
    );
  }

  void _handleCountdownTap(int index) {
    if (_countdownTimers[index] == null || !_countdownTimers[index]!.isActive) {
      startCountdownTimer(index);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Countdown Example'),
      ),
      body: ListView.builder(
        itemCount: 3,
        itemBuilder: (BuildContext context, int index) {
          return Card(
            margin: EdgeInsets.all(8.0),
            child: Padding(
              padding: EdgeInsets.all(16.0),
              child: Row(
                children: [
                  Expanded(
                    flex: 1,
                    child: Text(
                      'Breakdown ${index + 1}',
                      textAlign: TextAlign.center,
                    ),
                  ),
                  SizedBox(
                    height: 30,
                    child: GestureDetector(
                      onTap: () => _handleCountdownTap(index),
                      child: Container(
                        decoration: BoxDecoration(
                          color: Colors.blue,
                          borderRadius: BorderRadius.circular(8.0),
                        ),
                        child: Center(
                          child: Text(
                            '${_countdownTimes[index]} seconds',
                            style: TextStyle(
                              fontSize: 15,
                              color: Colors.white,
                            ),
                          ),
                        ),
                      ),
                    ),
                  ),
                ],
              ),
            ),
          );
        },
      ),
    );
  }
}