During the early days of this plugin, @fgrehm wrote a blog post with some benchmarks on the time that was cut down by using the plugin. If you are interested on the numbers only, the VMs tested were one of vagrant-lxc's Ubuntu dev boxes, rails-dev-box, his own rails-base-box and Discourse's dev box

First provision Second provision Diff. APT cache
rails-dev-box 4m45s 3m20s ~29% 66mb
rails-base-box 11m56s 7m54s ~34% 77mb
vagrant-lxc 10m16s 5m9s ~50% 124mb
discourse 1m41s 49s ~51% 62mb

_Please note that the tests were made on May 24th 2013 and nowadays they might be a bit different_
Some people have shared their numbers on Twitter and had experienced even better results:

Holy cow... If you dig Vagrant, and like time - you need Vagrant Cachier. 60% speed increase for me. @vagrantup

— Chris Rickard (@chrisrickard) November 12, 2013

vagrant-cachier saved 3:20 off my #vagrant #provisioning

— Joe Ferguson (@svpernova09) November 11, 2013

Tested vagrant-cachier. Saved 60% of vagrant up time installing 10 rpms with chef. Pretty awesome. Check it out!…

— Miguel. (@miguelcnf) June 9, 2013

vagrant-cachier took my vagrant spin up from 30 to 5 minutes and reduced my caffeine intake by 3 cups

— Russell Cardullo (@russellcardullo) June 7, 2013