For this study, I collected PAIR data from a sample of about 10,000 published utility patent applications that were filed 2007-2016. (not considering priority claims). For each of those applications, I simply categorized whether the application was (1) patented; (2) abandoned; or (3) still pending. The chart above shows the results grouped by quarterly filing date. Looking at applications filed in 2014q4 (~ two years ago) 26% are patented, 6% abandoned, and 68% still pending. By the time applications application have been pending for four years only about 15% remain pending.
There are few abandoned applications during the first 2 1/2 years of pendency – it seems that most applicants wait at least for a final rejection before giving up. An application pending for 4+ years is likely to involve substantial after-final practice (RCEs, appeals, etc.) that substantially slows down final resolution.