doc: adding estimated execution time

PR-URL: #41142
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Stewart X Addison <sxa@redhat.com>

doc/guides/contributing/pull-requests.md

@@ -251,7 +251,9 @@ And on Windows:
 > vcbuild test
 ```
 
-(See the [running tests][] section of Building guide for more details.)
+For some configurations, running all tests might take a long time (an hour or
+more). To run a subset of the test suite, see the [running tests][] section of
+the Building guide.
 
 ### Step 7: Push