Ubiquiti UniFi's currently have an option to create a guest hotspot network. By default, when using the vouchers, they only allow you to select a period of time, but not any MB or speed limits. Although this function is built into the units, they've yet to include it on the web interface. This is a simple hack that bypasses that.
Warning: This modification is not supported by Ubiquiti or Go Wireless Ltd and must be used at your own risk!
This is designed for UniFi version 2.4.6 (latest at time of writing)
Download the files from here
Locate where your UniFi software is installed, default is: C:\Users\%USERNAME%\Ubiquiti UniFi\
Unzip the file you've downloaded from the website and copy and paste the 'webapps' directory into the UniFi folder.
You should be prompted if you wish to replace all the files. Select yes.
Now your Hotspot vouchers page should contain a quota for MB, and limiting download and upload speed.
Ubuntu Server 12.04:
Open up your commend prompt
tar -xf UniFi-V.2.4.6.tar
cp unifi-hotspot.2.4.6.js /usr/lib/unifi/webapps/ROOT/library/js/unifi-hotspot.2.4.6.js
cp hotspot.jsp /usr/lib/unifi/webapps/ROOT/hotspot.jsp
cp voucher.jsp /usr/lib/unifi/webapps/ROOT/pages/voucher.jsp
Much Kudos for this hack to mopificius on the Ubiquiti forums