there is no timer for the entire extension to be entered, there's only an "inter digit" timer similar to Call Manager - by default this is 1500 ms - meaning it'll wait for 1 and a half seconds for you to hit another digit before it decides you're done and looks up what you've entered - if folks are pausing for longer than that between digits it'll time out.
you can adjust this on the caller input page (for both call handlers and users) - down at the bottom you'll see the 1500 default. You can crank it up but don't go too wild because then when someone enters some digits it will wait that long till it decides they're finished - so something like 5 seconds is way too long - don't do that.
You can change this in the template(s) for users and call handlers and you can bulk change it for existing handlers and users with the built in bulk editing capability - but I'd try it on a single user or handler first and find the time the customer likes...
