It would be significantly helpful if it were possible to determine the duration of recurring invoices based on the number of occurances/invoices generated, instead of only the End Date. As it stands now, it takes a good bit more work to have to manually count out the weekly or bi-weekly cycles to figure out which day to set as End Date, rather than just configuring it to run for a certain amount of cycles.
Also, there are often situations in which a recurring invoices need to be sent out in frquencies of less than 1 week or in frequences of an odd number of days. For instance, before switching to Invoice Ninja, we billed daily service in quantities of 15 days, and set the billing cycle to 15 days to coincide with the service that was being provided. Not being able to set up recurring cycles of 15 days has really made things difficult and required that we adjust significant parts of our workflow.