Since the rollout of the new gym system it is now only;
1. when it is convenient for me to take a gym (on the way to / from work),
2. When I want to build up my gym medal(s), or
3. if I feel like making the gyms near my home red, so mainly mornings & evenings. I'm not too worried about coins, gyms turn over too quickly to get too many. Also I don't go out on "gym runs" to take gyms down like I used to. I miss the prestige aspect of gyms - it was always fun to use Pokémon that were half as strong as the defenders to build the gym up.
I do like trolling people on gyms. I sat at one of my local gyms (which is in a busy shopping centre) within 2m of another trainer (who I'd never met & who didn't realise I was watching them attacking) and repeatedly took the gym down after they beat my mon. I kept putting in low CP amazing evolutions just for fun & after a couple of turns he did too. We continued for 15mins or so until I had to leave then I got up & told him I'd let him take the gym back & we had a laugh.
I normally use one of these - again all in good fun once people realise I'm baiting them.