Annual turnover is a good way to inspect the average time horizon a fund employs. Higher turnover levels will generally add to the expense ratio of a fund and could increase capital gains distributions.
Benchmark funds like the S&P 500-matching SPDRs have very low turnover, usually less than 10% per year. On the other end of the spectrum, some funds employ highly active trading strategies that push annual turnover past 100% per year (100% is a full turnover of the portfolio).