I'll give them a week off here or there becuase I get busy but I don't really ever give them a solid long break unless they're younger. It's better for them to stay in shape especially the older ones. Now I'm not talking ride hard everyday, a light week of riding is great, even just riding a few days a week. If you think about it, when older people stop moving and lay on the couch, that's when issues start happening and they lose muscle tone and it's harder to bring them back. For another example, athletes might take a break for the season but they don't stop working out completely. 

We are just about to the end of our barrel racing season here in NW Iowa. I usually give my horse a few months off December-Februrary then start legging her back up in mid-Feb. We might take the occasional trail ride here and there if the weather permits over the winter. We have an indoor that has a Fall barrel racing series with about 4 runs this fall ending in November and usually pick back up late March or April. Last year was the first time giving my mare a few months off and I really think it helped! (aside from a few trail rides here and there.)
She is 9 now and they were able to be out and just be a horse, get fat & hairy.

I have one rope horse that had been in training for a solid year.  She was working great.  After the February futurity I took her home and gave her 3 months off.  In June we got back on her and she was even better than before.  A break can do them good, both physically and mentally.  

To build muscle the horse needs quality proteins in the diet and that builds muscles in the back too.