The simple (although unhelpful) answer to your question is "No, we can't tell you". That's because we don't know what type of files you're planning on putting onto the memory stick (audio? video?) and what level of compression will have been used in compiling those files.
However a 2Gb memory stick (which is obviously what you've got) will typically hold around 500 3-minute MP3 tracks (when recorded at the level of compression normally used for MP3 downloads), so that's around 1500 minutes (or about 25 hours) of playing time.
Alternatively a 1½ hour movie download (in high definition, at commonly used compression ratios) might be around 1.7Gb, which would nearly fill the capacity of your memory stick. (So you'd only be able to get just over 1½ hours playing time if you could fill the whole of the available memory).