If Iphone 3GS cellular Data is turned on all the time, does it consume mb?

I have a package of 120mb free to roam internet. If i just have it ON all the time, does it consume my MB all this time, or when i receive a Skype or Whatsapp message only it consumes the transferring KB and then stop consuming again?

There are other things, such as AppStore, imessage, (and social updates like Facebook, Twitter, etc.) and plenty of OTHER apps that can consume some data in the background. Unfortunately, you'll just have to try it for a day or two and check your data usage per day and see how you stack up.

Yes, the phone will consume data even when you're not using it. Lots of apps are constantly updating and that uses data.

Plus you consume more battery power by leaving 'data' switched on, id turn data off when not in use, Wi-Fi uses less power than 3g/4g data ;-)