Hey Delinda,
The only issue I see with your idea is that you would need multiple lists for the amount of days. For example, if you are looking for Leads who have not been sent an email in the past 7 days, opposed to 10, they will all be part of the same list and you will have to have another filter clarifying when they were added to the list., Ot you will need a list for 1 day ago, 2 days ago, 3 days ago, etc. which doesn't sound like a whole lot of fun.
One idea would be to create a custom field in Marketo, something like "Last sent email date". Then, you can have one trigger campaign to update that field with the date whenever a Lead is sent an email. So when you go to filter based on last email sent (which could include Sales Emails as well), you can reference that field and only those with dates older than 7 days would qualify.