How much exactly you will have to spend at an online casino to become a VIP player depends on the casino operator. Many operators are transparent about how their reward system works. They may offer a levelling system or other type of progress bar, where you can easily track how much you've played already. You can also see how many points you've collected, and how much you'll have to play to reach the next reward or level.
However, some casinos don't disclose how much exactly you have to spend in order to become a member of the VIP club. They will send personal invitations to eligible members, or you can send them an e-mail and enquire about becoming a member.