Depends on the number of contacts. A good number would be the number of contacts you have times 90. So if you have 1000 contacts you'll be consuming ~85 KB every time you connect to Whatsapp servers.

In an official Whatsapp client would be that number every hour, so if you have 1000 contacts you'll be consuming around ~2 MB each day.

I have this idea about "Smart sync" to optimize that a little bit. Something like sync your whole address book once a day, and every hour sync just confirmed Whatsapp users (so people's status would be refreshed every hour). Maybe code a GUI to configure it even more. There are some people that just don't care about people's status and could turn that off or set it like once a day or something like that.

Other people just LOVE reading people's status like every minute and would love to sync even more often than an hour. That's why the option "Sync Now" was added, for those status freaks