How long will it be before someone writes a client sync portion (Windows, Mac and Linux) to make this work like Dropbox? At 50GB and 100GB this costs half of w

How long will it be before someone writes a client sync portion (Windows, Mac and Linux) to make this work like Dropbox? At 50GB and 100GB this costs half of what Dropbox costs. And it goes bigger. I'm currently at 70GB of Dropbox, and am wondering what happens when I hit 100GB.