How to search active relationships (e.g) during past year?
I would do the following, although it's a bit complicated so maybe you will want to develop a custom search after reading :).

Create a smart group to keep track of your active volunteers by searching for relationship type 'volunteer for' and status = active only (no start or end dates so you grab those empty end date people).

When you have your date criteria, for example the past year, create a group of folks who stopped volunteering in the past year by searching relationship type 'volunteer for', status = inactive only, and end date choose range choose your range, such as Jan 1, 2014 - Current date.

Now to get folks who are still active, or ended in the past year, do an OR search (checkbox in advanced search) using your 'active' smart group and your 'inactive in the past year' group.