Board Thread:Naruto Discussions/@comment-5273810-20150213005350/@comment-1018594-20150213073224

The First and Second Shinobi World Wars mirror World Wars I and II, so it was 20 years in-between. Dan was 27 during his death in the Second War, which makes sense: assuming him and Tsunade were the same age, Hashirama was dead before the First War and he lived to see Tsunade as a toddler.

There's honestly not enough information on the Second War to tell how long it was until the Third War, but it was enough for Jiraiya to train the Ame Orphans, train Minato, and then for Minato to become a Jonin and have his own team.

Kakashi and Obito were 13 during the Third Shinobi World War and 31 during the Fourth, so it's been 18 years, give or take a few months.