Right now when you go to your Filters page and select one to "Apply to existing messages", there is no way to subsequently tell that it is actively executing or when it is done executing.

If you go to the same or a different filter and select "Apply to existing messages" when the first filter is not done executing, all you get is the error notification "Another action is currently in progress. Try again later".

There is currently no way to tell when you can apply a second filter other than to keep clicking and hoping.

One solution would be to show on this screen any filters that have been applied and are currently processing emails.

Another would be to just let us apply multiple filters for active processing simultaneously.

UPDATE — It seems like even when you are finally able to apply a subsequent filter that the first filter may still be processing?