For me, I'm trying to figure it out. Right now it's with my 25 (soon to be 26) year old Half-Arab mare who hasn't really been worked since May, maybe even longer. I don't want to push her too hard with her getting up there in age, but I do want to bring her back into a healthy condition slowly.
Starting today I rode approx. 20 minutes, with around 5 being a trot. We did do maybe 3 minutes of hill work too. As she is able to hold herself longer, we will increase more and more. But for now this is what she needs.