If you're not on a schedule like that, then you're almost certainly trying to plan your ride: maybe it's fitness sets, a specific exercise that you don't want to let go too long, or just a time by which you have to be out of the ring to make way for something else.
I've struggled on and off over the years with timing myself at the barn, and I've never come up with a really good solution.
Currently, I use a combination of two things: the clock in the indoor and a sports watch.
The current sports watch.
The clock at the indoor is useful: it's semi-prominently displayed, large, and it lets me break up my work into rough five minute segments, or keep track of a longer ride. It's not precise, though, and obviously it's on the wall. I try to spend as little time as possible in the indoor during good weather, so that doesn't help when I'm outside.
The sports watch is useful because it is water resistant, so I don't have to worry about taking it off before bathing or hosing Tris down. It has a basic stopwatch function that can be useful for trot sets, except I have to keep looking at it constantly. The one pictured above is super-basic and cheap. I think it cost around $15. Its downside is that I have yet to find one that really holds up well to barn work. They tend to crap out after a few years - dust in the gears, just planned obsolescence, some combination of that.
For a time, I experimented with using my iPhone to track trot sets. I wanted something that was brainless, so I set up a series of staggered alarms for what I was doing. 5 minutes trot, 3 minutes rest, so on and so forth. It gave me a certain peace and worked really well, but it was a complete PITA to set up. That was before there were a lot of interval apps for the iPhone - hopefully something like that would be easier than setting all those alarms!
On the other hand, the iPhone was fragile and bulky to carry, so frustrating.
Anyone have a better solution? What do you use to keep track of time? What about timing fitness work?