My understanding is that they are only valid for three months from complete reading. I have several odd cases in my tree.
1. banns were fully read in one parish where both resided, then read again starting almost immediately in another parish a long way away (again both said to be resident there), followed by marriage in the second parish. The groom was a soldier and it looked like he was posted suddenly while the first set where being read. The bride may have been the daughter of another soldier, but not sure.
2. Banns read in 1830s, but no marriage until 1860s. On censuses between these dates, couple lived together producing three children. He said he was married, she said she was his single "housekeeper". So I suspect someone objected to the original banns because they knew he was already married.
3 Banns read twice about 18 months apart in two different parishes, couple married straight after 2nd set. I think bride was under age when first set read, didn't do any further research as they turned out not to be related just a co-incidence of names.
If you don't find a second set of Banns or a licence corresponding to the actual marriage, it is possible that the vicar was ignorant of the fact that Banns had a "use by" date
