Note HOMEBREW_NO_INSTALL_CLEANUP in the help/manual

Library/Homebrew/cmd/install.rb

@@ -20,6 +20,8 @@ def install_args
 
         <formula> is usually the name of the formula to install, but it can be specified
         in several different ways.
+
+        Unless `HOMEBREW_NO_INSTALL_CLEANUP` is set, `brew cleanup` will be run for the installed formulae or, every 30 days, for all formulae.
       EOS
 
       switch :debug,

Library/Homebrew/cmd/reinstall.rb

@@ -14,6 +14,8 @@ def reinstall_args
         `reinstall` [<options>] <formula>
 
         Uninstall and then install <formula> (with existing and any appended install options).
+
+        Unless `HOMEBREW_NO_INSTALL_CLEANUP` is set, `brew cleanup` will be run for the reinstalled formulae or, every 30 days, for all formulae.
       EOS
       switch :debug,
         description: "If brewing fails, open an interactive debugging session with access to IRB "\

Library/Homebrew/cmd/upgrade.rb

@@ -18,6 +18,8 @@ def upgrade_args
 
         If <formula> are given, upgrade only the specified brews (unless they
         are pinned; see `pin`, `unpin`).
+
+        Unless `HOMEBREW_NO_INSTALL_CLEANUP` is set, `brew cleanup` will be run for the upgraded formulae or, every 30 days, for all formulae.
       EOS
       switch :debug,
         description: "If brewing fails, open an interactive debugging session with access to IRB "\

Library/Homebrew/manpages/brew.1.md.erb

@@ -238,8 +238,8 @@ Note that environment variables must have a value set to be detected. For exampl
 
   * `HOMEBREW_NO_INSTALL_CLEANUP`:
     If set, `brew install`, `brew upgrade` and `brew reinstall` will never
-    automatically remove the previously installed version(s) of the
-    installed/upgraded formulae.
+    automatically cleanup the installed/upgraded/reinstalled formulae or all
+    formulae every 30 days.
 
   * `HOMEBREW_PRY`:
     If set, Homebrew will use Pry for the `brew irb` command.

docs/Manpage.md

@@ -217,6 +217,8 @@ Install *`formula`*.
 *`formula`* is usually the name of the formula to install, but it can be specified
 in several different ways.
 
+Unless `HOMEBREW_NO_INSTALL_CLEANUP` is set, `brew cleanup` will be run for the installed formulae or, every 30 days, for all formulae.
+
 * `--env`:
   If `std` is passed, use the standard build environment instead of superenv.If `super` is passed, use superenv even if the formula specifies the standard build environment.
 * `--ignore-dependencies`:
@@ -362,6 +364,8 @@ all formulae or to determine if any current formulae have Ruby issues.
 
 Uninstall and then install *`formula`* (with existing and any appended install options).
 
+Unless `HOMEBREW_NO_INSTALL_CLEANUP` is set, `brew cleanup` will be run for the reinstalled formulae or, every 30 days, for all formulae.
+
 * `-s`, `--build-from-source`:
   Compile *`formula`* from source even if a bottle is available.
 * `--force-bottle`:
@@ -538,6 +542,8 @@ Upgrade outdated, unpinned brews (with existing and any appended install options
 If *`formula`* are given, upgrade only the specified brews (unless they
 are pinned; see `pin`, `unpin`).
 
+Unless `HOMEBREW_NO_INSTALL_CLEANUP` is set, `brew cleanup` will be run for the upgraded formulae or, every 30 days, for all formulae.
+
 * `-s`, `--build-from-source`:
   Compile *`formula`* from source even if a bottle is available.
 * `--force-bottle`:
@@ -1107,8 +1113,8 @@ Note that environment variables must have a value set to be detected. For exampl
 
   * `HOMEBREW_NO_INSTALL_CLEANUP`:
     If set, `brew install`, `brew upgrade` and `brew reinstall` will never
-    automatically remove the previously installed version(s) of the
-    installed/upgraded formulae.
+    automatically cleanup the installed/upgraded/reinstalled formulae or all
+    formulae every 30 days.
 
   * `HOMEBREW_PRY`:
     If set, Homebrew will use Pry for the `brew irb` command.

manpages/brew.1

@@ -259,6 +259,9 @@ Install \fIformula\fR\.
 .P
 \fIformula\fR is usually the name of the formula to install, but it can be specified in several different ways\.
 .
+.P
+Unless \fBHOMEBREW_NO_INSTALL_CLEANUP\fR is set, \fBbrew cleanup\fR will be run for the installed formulae or, every 30 days, for all formulae\.
+.
 .TP
 \fB\-\-env\fR
 If \fBstd\fR is passed, use the standard build environment instead of superenv\.If \fBsuper\fR is passed, use superenv even if the formula specifies the standard build environment\.
@@ -453,6 +456,9 @@ Syntax\-check all of Homebrew\'s Ruby files\.
 .SS "\fBreinstall\fR [\fIoptions\fR] \fIformula\fR"
 Uninstall and then install \fIformula\fR (with existing and any appended install options)\.
 .
+.P
+Unless \fBHOMEBREW_NO_INSTALL_CLEANUP\fR is set, \fBbrew cleanup\fR will be run for the reinstalled formulae or, every 30 days, for all formulae\.
+.
 .TP
 \fB\-s\fR, \fB\-\-build\-from\-source\fR
 Compile \fIformula\fR from source even if a bottle is available\.
@@ -652,6 +658,9 @@ Upgrade outdated, unpinned brews (with existing and any appended install options
 .P
 If \fIformula\fR are given, upgrade only the specified brews (unless they are pinned; see \fBpin\fR, \fBunpin\fR)\.
 .
+.P
+Unless \fBHOMEBREW_NO_INSTALL_CLEANUP\fR is set, \fBbrew cleanup\fR will be run for the upgraded formulae or, every 30 days, for all formulae\.
+.
 .TP
 \fB\-s\fR, \fB\-\-build\-from\-source\fR
 Compile \fIformula\fR from source even if a bottle is available\.
@@ -1336,7 +1345,7 @@ If set, Homebrew will not use the GitHub API, e\.g\. for searches or fetching re
 .
 .TP
 \fBHOMEBREW_NO_INSTALL_CLEANUP\fR
-If set, \fBbrew install\fR, \fBbrew upgrade\fR and \fBbrew reinstall\fR will never automatically remove the previously installed version(s) of the installed/upgraded formulae\.
+If set, \fBbrew install\fR, \fBbrew upgrade\fR and \fBbrew reinstall\fR will never automatically cleanup the installed/upgraded/reinstalled formulae or all formulae every 30 days\.
 .
 .TP
 \fBHOMEBREW_PRY\fR