GUI/Intro: Estimate max age of backups that can be restored with pruning

src/qt/forms/intro.ui

@@ -219,6 +219,13 @@
        </property>
       </widget>
      </item>
+     <item>
+      <widget class="QLabel" name="lblPruneSuffix">
+       <property name="buddy">
+        <cstring>pruneGB</cstring>
+       </property>
+      </widget>
+     </item>
      <item>
       <spacer name="horizontalSpacer_2">
        <property name="orientation">

src/qt/intro.cpp

@@ -148,6 +148,7 @@ Intro::Intro(QWidget *parent, int64_t blockchain_size_gb, int64_t chain_state_si
     }
     ui->pruneGB->setValue(m_prune_target_gb);
     ui->pruneGB->setToolTip(ui->prune->toolTip());
+    ui->lblPruneSuffix->setToolTip(ui->prune->toolTip());
     UpdatePruneLabels(ui->prune->isChecked());
 
     connect(ui->prune, &QCheckBox::toggled, [this](bool prune_checked) {
@@ -381,6 +382,10 @@ void Intro::UpdatePruneLabels(bool prune_checked)
     }
     ui->lblExplanation3->setVisible(prune_checked);
     ui->pruneGB->setEnabled(prune_checked);
+    static constexpr uint64_t nPowTargetSpacing = 10 * 60;  // from chainparams, which we don't have at this stage
+    static constexpr uint32_t expected_block_data_size = 2250000;  // includes undo data
+    const uint64_t expected_backup_days = m_prune_target_gb * 1e9 / (uint64_t(expected_block_data_size) * 86400 / nPowTargetSpacing);
+    ui->lblPruneSuffix->setText(tr("(sufficient to restore backups %n day(s) old)", "block chain pruning", expected_backup_days));
     ui->sizeWarningLabel->setText(
         tr("%1 will download and store a copy of the Bitcoin block chain.").arg(PACKAGE_NAME) + " " +
         storageRequiresMsg.arg(m_required_space_gb) + " " +