There is one programme that I use, which gives some indication to download volume, and it is NetMedic. I cannot give a link as it appears that NetMedic is no longer available, but there is a similar programme which I think as now been substituted for NetMedic,I am sure someone may have the link to this, possibly Stuartli.
You can also check via your ISP account, but if the method is the same that is provided by my ISP Tiscali, then it can get a little complicated. You will need to check how many bytes you have used, then convert to GB. 1GB=1024mb.