Let's say "too long" is a day.
Run a filter for something like "updated < -1d and resolution = unresolved". The will give you a list of anything that's open and not updated in the last day, so it's probably going to be quite a long list, but I'm aiming to give you a rough idea, not an absolute. It's a query you should heavily edit to suit you and your users.
Save the filter, then go into the options and subscribe a group to it. The members of the group will be emailed the list on the schedule you set.
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