While all Orders in the system must currently have an estimated end date, you have a couple of options when are you are not sure when it may end.
Using Cycle Auto-Extend
A cycle bill is a long term rental that creates one Invoice per monthly period. You can set these to automatically extend out the dates once an Invoice is created. Please see this article in order to have cycle billing orders automatically extend dates.
Manually Extending Dates
When creating the order, put the end date out for a far out future date.
As this date approaches, you may notice it in your yard as "Coming In." Eventually, it may even be marked as overdue.
At this point, enter the order page and "Edit Dates" extend the dates out.