The ATF has released an official chart with estimated processing times for various NFA Forms. The current chart estimates Form 4s at 9 months as of December 2013. The chart doesn’t differentiate between eForms or paper filed forms but I’d have to assume they’re referring to paper forms; either that or they’re making no differentiation in their estimates. Here’s a link you can follow to take a look at the chart for yourself: Processing Times
You can also find the chart on the ATF's website under Firearms and clicking the Firearms News tab.
For my customers waiting on Form 4s submitted last year I’ve been receiving back paperwork around the 10 month mark.