Each forum page is around 100 kb, so if you load 20 pages an hour, that's about 2 MB per hour. Images will add a lot to that though, so let's say 5 MB. A short 5 minute Youtube video will, depending on the quality you choose, be around 15-16 MB. A longer 30 minute video is therefore about 90-100 MB. The Elements game client is only 4 MB and is cached, and the database updates are very lightweight, so it'll probably be less than 1 MB per hour. Omegle will be quite a bit more - probably around 10 MB per hour. Mangafox images are pretty small - around 150-200 kb each, so if you load 50 images each hour, that's around 10 MB.
So, to summarize, some very rough estimates:
Website | Estimated usage/hour |
Youtube | 200 MB |
Forum | 5 MB |
Elements | 1 MB |
Omegle | 10 MB |
Mangafox | 10 MB |
So, basically, try to avoid videos where you can or at least watch them in low quality and you should be good. If you do that you should be able to keep your usage around 10 MB/hour. Oh, and if you find you keep going over your limit, most websites offer a low-bandwidth version,
this forum being one of them. Good luck!