My colleagues at the Wesleyan Media Project--run by Erika Fowler, Mike Franz, and Travis Ridout (all Wisconsin grads, I might add)--provided some data to help answer that question. In 2010, the first election of the post-Citizens United era, four senators running for reelection faced competitive races: Senators Feingold, Reid, Murray, and Bennet. Feingold lost reelection, and the other three won with less than 53% of the vote. Here's when each launched their first ad:
Bennet: March 17
Feingold: April 27
Reid: April 30
Murray: July 28
Tester's first ad launched on March 26. I'd say given that he's a first-term senator running for reelection, his situation is most analogous to Bennet's--who was appointed to a vacant seat and running his first campaign. Murray and Feingold were both elected in 1992, Reid was elected in 1986--so well-established reputations. Four data points are not much to draw any firm conclusions, but it would seem that Tester's ad launch is not particularly early given the competitive nature of the seat and the fact that he's running for his second term. It seems just right.