For me, its been a very painful process as I had interviews at a handful of exceedingly good firms last time and it is already my third cycle. I feel like my prospects are shrinking every year! But I'm going to keep on pushing as I think once you've produced the best app you can, the quality of the competition and whether it will be accepted is out of your control, so you just have to remember it is as much a numbers game as anything.
I wouldn't view summer job apps as a proper 'cycle' that all firms do, because it really just depends on how each one views direct TCs in general. Some will have their direct TC window open from October and keep it open till July the following year and use the summer window to fill remaining gaps left by the vac schemes. Others will close their direct TC apps earlier, around December or Jan maybe, and reopen them between June-July to capture the window of talent that might have been too busy with exams to apply to vacation schemes earlier in the year, or in different stages of their life. Some firms only recruit in the summer. It really just depends on what you're dealing with.