SQL Help - Find number of days it took to pay off an invoice

SQL Help - Find number of days it took to pay off an invoice

I would like to collect data on how long it takes to pay off invoices. Ultimately, it will be aggregated by customer but for now should return each invoice.

For example I would like to see results along the lines of:

Invoice ID - Days to pay 
123456789 - 26

If the invoice is still open it would compare the invoice date with todays date for the difference.

Sellers