How do you track ongoing support epics?

[Jon Kruger]

We are starting to use Portfolio and I'm hoping that it helps us get better at planning for the future.  We are t-shirt sizing epics, and that's pretty straightforward to plan out in Portfolio, but what do you do with ongoing maintenance/support?  That "epic" (it's not really an epic) has no end date, but I want to allocate a certain percentage of capacity to it each sprint.  That percentage will change over time, for example, it could be 20% of capacity for awhile but next year we might spend a couple months doing 50% maintenance/support.

I'm looking for suggestions of how you all plan out future sprints in Portfolio when you have a combination of sized epics and percentage allocations to ongoing support efforts.