My Lumina 920 was lucky to last 10 hours with limited use. No way it would do multiple days.
Are you talking about the Lumia line? Getting 2 to 3 days of battery life on them is do-able, but you'll be making a lot of concessions. Plan on plugging in nightly. Anandtech does have Lumia's in their battery life charts, though those are worst case scenario.
Is this true that their battery life is so much better than the likes of iPhone or Galaxy S4 which needs to be charged at least twice a day if you're using it heavily or BlackBerry which needs to be charged at least once a day?
I really would like to buy a phone that I can use heavily chatting on whatsapp without being so paranoid about my battery depleting
and how is whatsapp on nokia phones anyway?
When I still used it, I only needed to charge my Nokia (E75) once every 4-5 days. Whatsapp is working perfectly without a problem. You simply download the app from here: http://www.whatsapp.com/nokia/
(I know this is probably not what you meant, but you asked about Nokia and Whatsapp, so I answered)
Are you running Anna, or Belle?
. . . and I have turned off certain things I don't use like Bluetooth and Wi-Fi unless I'm actively using them.