Well Actually, you can create a LOW Priority in SLA with the time-line of 1 Month for such Tickets and maybe increase the Due By date manually from the Tickets page. We are also coming up with more time/days option,e.g. 3 Months, 6 Months & 1 Year in the list.
One more thing you can do to override the SLA alerts in such cases would be, create a Custom Status in Admin -> Ticket Fields -> Status option with the SLA Timer OFF and when such tickets are submitted in your Freshdesk, which needs to be addressed in a long-cycle, just set this Status. E.g. in our Freshdesk, we have a status - Awaiting Developer Fix. Even though a Ticket will be in Medium or High priorty, we set the Status as Awaiting Developer fix, so we don't get SLA escalations for the Tickets
Hope this helps
Vijay