I purchased a cheap web hosting package with unlimited storage space and monthly transfer to host my website and blogs. I know the "unlimited" things is just a marketing gimmick and I am OK with that. So, according to your experience, how many GBs is the maximum allowed usage of an "unlimited monthly transfer"?

They all differ and may in fact decide upon 'fair usage' and advertise that amount within the small print. Check out your terms and conditions to see if there is anything there.

The TOS can be tricky from looking around at a couple of places but I am not doing too bad so far. I think it depends on what you want to have on your site too as some file types are "bigger" than others.

"Fair usage" sounds unfair. It implies that they can use their own judgment to decide up to how much bandwidth you can use before you are considered exceeding the bandwidth limit. I would rather go for hosting packages where the quotas are clearly mentioned.