My hairstylist is always telling me that I need to get my haircut every 2-3 months, even with my long straight hair. I always ignore this advice and only get it cut every 6 months, worth it for a savings of $320 a year.
To stretch my cuts, I get a very basic cut, side swept bangs that blend in as they grow out, and soft long layers. If I notice split ends, I rub a dab of olive oil on them to smooth them out. And will do some leave-in conditioner treatments to keep ends silky. 6 months between cuts works fine for me.