This will depend on the data being exchanged. If transferring files then the size will tell you if you can use this dongle. If using applications, it will depend on the application.
In any case, you can view the statistics of the VPN connection as you use it to determine how much data is being exchanged.
Give it a try and monitor the connection.
300MB in 4 minutes seems very high. Given your concern over use, make sure you have any background apps/servcies that might use bandwidth are off. Things like Skype and any torrent should be disabled. Auto updates for Windows or AV also.