demux: bump the demuxer cache readahead duration

Set it to 10 hours, which is practically unlimited. (Avoiding use of
"inf", since that might interact strangely with the option parser and
such.)

DOCS/man/options.rst

@@ -3835,7 +3835,9 @@ Cache
 ``--cache-secs=<seconds>``
     How many seconds of audio/video to prefetch if the cache is active. This
     overrides the ``--demuxer-readahead-secs`` option if and only if the cache
-    is enabled and the value is larger. (Default: 120.)
+    is enabled and the value is larger. The default value is set to something
+    very high, so the actually achieved readahead will usually be limited by
+    the value of the ``--demuxer-max-bytes`` option.
 
 ``--cache-pause``, ``--no-cache-pause``
     Whether the player should automatically pause when the cache runs low,

demux/demux.c

@@ -114,7 +114,7 @@ const struct m_sub_options demux_conf = {
         .max_bytes = 400 * 1024 * 1024,
         .max_bytes_bw = 400 * 1024 * 1024,
         .min_secs = 1.0,
-        .min_secs_cache = 120.0,
+        .min_secs_cache = 10.0 * 60 * 60,
         .seekable_cache = -1,
         .access_references = 1,
     },