Right.. I believe we get 20MB daily for goodybags so how much is 20MB as in how many minutes can I browse the net for.. how many songs I can download etc.. ?
Browsing is not usefully measured in time and notoriously difficult to even measure in pages due to variable content and compression.
20MB would probably not be enough to download a whole album of mp3s, maybe half an album. Maybe 5 mins of streaming video (depends on what service)
If you have a goodybag with unlimited data then there is no actual limit, just don't use excessive amounts otherwise you could be barred. If you don't have unlimited data then there is still no limit on what you can use, it is 20p for up to 20mb each day but after the first 20mb you will be charged per mb used - better to get an unlimited goodybag if you use data on a regular basis.
as for what 20mb allows you to do..... well there are too many variables to answer this, some websites use a lot more data than others, also mobile websites use less data than the full website, many apps use data even when you're not using them, some apps use more data than other apps. Basically 20mb won't allow you to do very much!