I'm not an expert by any means, however I would say humans are definitely up there with a bit less than a quarter of the average lifespan (18 of 80).
Other's to consider in my opinion would be orcas, males are known to live around 20 years, and females 45 on average. Orcas finish weaning at around 2 years, but aren't sexually matured until 10–15 years. Its difficult to pinpoint the end of child rearing due to the close relationship in pods, as they never leave.
Chimpanzees live around 30 years in the wild (much longer in captivity) and reach sexual maturity at around 7 years of age, which is a decent chunk of time.
Elephants live approximately 50–70 years depending on the species. They finish weaning between 5–10 years, and the adolescent stage ends around 17 years, and sexual maturity happens within that stage.
I figure that sexual maturity is a good estimate of the end of child rearing, at lest for social animals, it signifies full adulthood, and competition in mating.
Some good answers already.
Humans do spend 18 of their 75 years raising a single individual and they tend to keep coming back to the nest for holidays, long illness, lose job, etc.
many small animals, ill chose Cavy’s since I know about them. At 3 to 3.5 months the female can become pregnant for 68–72 days then have a live birth. They are fertal and will mate again an hour after the last baby is out. Typically babies stay with the mom and Dad for 8 weeks to learn how to be a Cavy. A new litter arrives and repeats. So except for the first three months a female may have babies until they pass. Average lifespan is 6 years so (72-3)/72= 96% of a female cavys life could be spent pregnant.
That said, if cavys are allowed to get pregnant one after another it greatly reduces he lifespan. When I was 5 my first two kept having babies and I had no idea why. The female only lived 4.2 years because the physical demands of making the babies. The male lived to be 9!