Those totals for first two quarters of this year blew away the quarterly three-year average of flows into TDFs, which Morningstar puts at $13.6 billion.

That suggests market saturation of TDFs may still be some way off.

In 2006, less than a third of plan sponsors offered TDFs, according to the Investment Company Institute. By 2012, almost 70 percent of plans offered TDFs.

But availability and uptake are different measures. TDFs accounted for a mere 3 percent of plan assets in 2006, before the passage of the PPA that year. By 2012, even though most plans offered TDFs, the funds only accounted for 13 percent of assets.

Though that's a substantial increase, the numbers suggest there is still plenty of room to grow.

In 2012, ICI said 46 percent of plan assets were held in mutual funds—more than three times the total held in TDFs.

Many analysts are expecting a shift in that portion going forward. Perhaps the boldest prediction came from Cerulli Associates at the end of last year, when the Boston-based data company projected TDFs will capture 90 percent of all 401(k) assets by 2019.

In its report, Morningstar puts the total value of TDFs at $760 billion. Others claim more. In any case, what is clear is that sponsors and plan participants continue to leverage the value of the professional risk management that TDFs offer in 401(k) savings strategies.

Janet Yang, director of research for multi-asset strategies at Morningstar, called target-date funds "among the most effectively used instruments in the fund industry."

That effectiveness has encouraged increased competition among fund managers, a reality Jessica Sclafani, a senior analyst at Cerulli, expects will intensify in the near future, according to a statement she made when the firm released its data late last year.

That means sponsors and participants should expect to have a wider choice of TDF options.

While proponents of competition would see that as a net positive, it also suggests that advisors to retirement plans will have to step up to help guide sponsors through the growing product landscape in order to deliver best-in-class solutions for participants.

Already, TDFs vary widely in design and risk allocation, even when comparing products with the same glide path.

One RIA recently documented a wide variance in risk exposure between competing funds with the same target date.

Equity holdings ranged from 20 percent to 70 percent of assets as the funds approached and reached their target date, according to data cited by Ron Surz, president of consultancy Target Date Solutions in Orange County, Calif. That proves not all TDFs are the same, and suggests that as the TDF market grows, so will the plan advisor's role in helping sponsors determine the risk appetite of participants.

In its breakdown of last quarter's numbers, Morningstar also touched on the growing variety in the TDF product landscape.

In 2014, the average TDF with a 2050 glide path held about a 90 percent stake in equities. The average fund with a 2015 glide path held about 40 percent in equities.

Yet the two groups of funds experienced similar returns last year—the 2050 series averaged about 1 percent more in returns than the 2015 class—even though equities experienced strong returns in 2014.

To Yang and the analysts at Morningstar, the fact that there wasn't a wider difference in return between the two target-date years indicates "widely varying approaches" to managing target-date assets.

That may just underscore the fact that the importance of a plan advisor's role grows as more TDF options emerge and more retirement assets flow their way.