diff --git a/broker/append_fsm.go b/broker/append_fsm.go
index f8b49023..68ea372c 100644
--- a/broker/append_fsm.go
+++ b/broker/append_fsm.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"hash"
 	"io"
+	"time"
 
 	"github.com/pkg/errors"
 	log "github.com/sirupsen/logrus"
@@ -435,9 +436,26 @@ func (b *appendFSM) onValidatePreconditions() {
 	// reflected in our index, if a commit wasn't accepted by all peers.
 	// Such writes are reported as failed to the client and are retried
 	// (this failure mode is what makes journals at-least-once).
-	var indexMin, indexMax, indexDirty = b.resolved.replica.index.OffsetRange()
+	var indexMin, indexMax, indexModTime = b.resolved.replica.index.Summary()
 	var suspend = b.resolved.journalSpec.Suspend
 
+	// The index is "clean" if all fragments have been remote for the journal's
+	// flush interval, where the flush interval is interpreted as an upper-bound
+	// expectation of the period between appends if the journal remains "in use".
+	// Thus, if a journal doesn't recieve an append for more than its interval,
+	// it's presumed to be idle and is eligible for suspension.
+	//
+	// To see why, consider a group of journals which are appended to at midnight,
+	// configured with a 24h flush interval. These journals will not auto-suspend
+	// ordinarily. If we instead used a more aggressive policy, they might trigger
+	// storms of suspensions and re-activations which could impact other journals
+	// due to assignment churn.
+	var flushInterval = int64(b.resolved.journalSpec.Fragment.FlushInterval.Seconds())
+	if flushInterval == 0 {
+		flushInterval = 24 * 60 * 60 // Default to 24h.
+	}
+	var indexDirty = indexModTime != 0 && indexModTime < time.Now().Unix()-flushInterval
+
 	var maxOffset = b.pln.spool.End
 	if indexMax > maxOffset {
 		maxOffset = indexMax
diff --git a/broker/fragment/index.go b/broker/fragment/index.go
index 8cdc7ca2..f6be5618 100644
--- a/broker/fragment/index.go
+++ b/broker/fragment/index.go
@@ -125,13 +125,17 @@ func (fi *Index) Query(ctx context.Context, req *pb.ReadRequest) (*pb.ReadRespon
 	}
 }
 
-// OffsetRange returns the [Begin, End) offset range of all Fragments in the index,
-// and a boolean indicating if the index has local-only fragments.
-func (fi *Index) OffsetRange() (int64, int64, bool) {
+// Summary returns the [Begin, End) offset range of all Fragments in the index,
+// and the persisted ModTime of the last Fragment (or zero, if it's local).
+func (fi *Index) Summary() (int64, int64, int64) {
 	defer fi.mu.RUnlock()
 	fi.mu.RLock()
 
-	return fi.set.BeginOffset(), fi.set.EndOffset(), len(fi.local) != 0
+	if l := len(fi.set); l == 0 {
+		return 0, 0, 0
+	} else {
+		return fi.set[0].Begin, fi.set[l-1].End, fi.set[l-1].ModTime
+	}
 }
 
 // SpoolCommit adds local Spool Fragment |frag| to the index.
diff --git a/broker/fragment/index_test.go b/broker/fragment/index_test.go
index 9a4d6c08..b70add4c 100644
--- a/broker/fragment/index_test.go
+++ b/broker/fragment/index_test.go
@@ -288,7 +288,7 @@ func (s *IndexSuite) TestWalkStoresAndURLSigning(c *gc.C) {
 	<-ind.FirstRefreshCh()
 
 	c.Check(ind.set, gc.HasLen, 3)
-	var bo, eo, _ = ind.OffsetRange()
+	var bo, eo, _ = ind.Summary()
 	c.Check(bo, gc.Equals, int64(0x0))
 	c.Check(eo, gc.Equals, int64(0x255))
 
@@ -306,7 +306,7 @@ func (s *IndexSuite) TestWalkStoresAndURLSigning(c *gc.C) {
 	ind.ReplaceRemote(set)
 
 	c.Check(ind.set, gc.HasLen, 4) // Combined Fragments are reflected.
-	bo, eo, _ = ind.OffsetRange()
+	bo, eo, _ = ind.Summary()
 	c.Check(bo, gc.Equals, int64(0x0))
 	c.Check(eo, gc.Equals, int64(0x555))
 
diff --git a/broker/replicate_api_test.go b/broker/replicate_api_test.go
index 637a3f21..65182ed2 100644
--- a/broker/replicate_api_test.go
+++ b/broker/replicate_api_test.go
@@ -40,7 +40,7 @@ func TestReplicateStreamAndCommit(t *testing.T) {
 	require.NoError(t, stream.Send(&pb.ReplicateRequest{Content: []byte("bazbing"), ContentDelta: 6}))
 
 	// Precondition: content not observable in the Fragment index.
-	var _, eo, _ = broker.replica("a/journal").index.OffsetRange()
+	var _, eo, _ = broker.replica("a/journal").index.Summary()
 	require.Equal(t, int64(0), eo)
 
 	// Commit.
@@ -58,7 +58,7 @@ func TestReplicateStreamAndCommit(t *testing.T) {
 	expectReplResponse(t, stream, &pb.ReplicateResponse{Status: pb.Status_OK})
 
 	// Post-condition: content is now observable.
-	_, eo, _ = broker.replica("a/journal").index.OffsetRange()
+	_, eo, _ = broker.replica("a/journal").index.Summary()
 	require.Equal(t, int64(13), eo)
 
 	// Send EOF and expect its returned.