My way of showing a paywall every day for a few days, then every week, then every two weeks. Good for about a year.

paywall_showing_schedule.dart

import 'package:collection/collection.dart';
import 'package:shared_preferences/shared_preferences.dart';

import '/architecture/let.dart';
import '/membership/membership_state.dart';
import '/membership/membership_tasks.dart';
import '/telemetry/analytics.dart';

Future<void> appOpenAction() async {
  $analytics.vglAppOpenActionRequested();

  final prefs = await SharedPreferences.getInstance();

  final now = DateTime.now();
  final first = prefs.firstDate ??= now;
  final last = prefs.lastDate ?? DateTime(2020);
  final today = now.difference(first).inDays;
  final lastDay = last.difference(first).inDays;
  final nextDay = schedule.next(lastDay);

  // We've completed the entire schedule!
  if (nextDay == null || nextDay > today) {
    $analytics.vglAppOpenActionSkip();
  } else {
    $analytics.vglAppOpenActionShown();
    prefs.lastDate = now;
    _handleAppOpenAction();
  }
}

void _handleAppOpenAction() {
  if (!$revenueCat.proOrGrandfathered) {
    showPaywall(PaywallReferrer.appOpen);
  }
}

Iterable<int> get schedule sync* {
  var x = -1;

  for (var i = 1; i < 30; i++) {
    switch (i) {
      case <= 4:
        x += 1;
      case <= 7:
        x += 4;
      case <= 14:
        x += 7;
      case _:
        x += 14;
    }
    yield x;
  }
}

extension on SharedPreferences {
  DateTime? get firstDate => getString('la-fd')?.let(DateTime.tryParse);
  set firstDate(DateTime? x) => setString('la-fd', x?.toIso8601String() ?? '');
  DateTime? get lastDate => getString('la-ld')?.let(DateTime.tryParse);
  set lastDate(DateTime? x) => setString('la-ld', x?.toIso8601String() ?? '');

  Future<void> clearDates() async {
    await remove('la-fd');
    await remove('la-ld');
  }
}

Future<void> clearAppOpenActionDates() =>
    SharedPreferences.getInstance().then((x) => x.clearDates());

extension on Iterable<int> {
  Iterable<(int, int)> get pairwise sync* {
    for (var i = 0; i < length - 1; i++) {
      yield (elementAt(i), elementAt(i + 1));
    }
  }

  int? next(int needle) {
    final s = sorted((a, z) => a - z);
    for (final (i, (a, b)) in s.pairwise.indexed) {
      if (i == 0 && needle < a) {
        return a;
      }
      if (a <= needle && needle < b) {
        return b;
      }
    }
    return null;
  }
}