My sister thinks that my horse is pining. Honestly it hasn't been long since I last saw her, probably just twice as long as between normal visits since my work went pear shaped.

So I don't think it's pining but maybe she is homesick, or just aware of the changes and people worrying about her.

How long is she likely to need to get over this?

Its very possible she is pining. She may or may not get over it by you going to visit her. Some horses are exceptionally tuned to their owners visiting regularly others aren't. She no doubt notices the difference in schedule.

Maybe she misses you. I know when I went on vacation my dog spent a good part of it pining for me. I don't think we give out animal friends enough credit. They can feel lonesome too.