The Economist has an interesting graphic on the number of days before a US presidential election where a candidate announces his/her intention to run.
Interestingly, Hillary Clinton's announcement this past Sunday, 577 days before the 2016 election, is later than when she announced back in 2008.
She seems to be in a stronger position now than back in 2008. The lesson from 2008 seems to be to not announce so early that you give other Democrat contenders adequate time to come up with a credible strategy like Obama did during that hotly contested election.
This time around, if other Democrat contenders don't announce soon, they might not have enough campaign finance and political strategist to battle the former First Lady/US Senator/Madam Secretary (just the listing of her former titles is impressive).