Currently, the network is growing at around 1GB per month. It's hard to anticipate how large it could grow and at what pace, but there are already efforts underway to implement state-tree pruning in various clients. These techniques will contribute towards so-called "light-clients".
In linked chart above, Block size evolution estimating block size in bytes over time that can be used to estimate velocity (rate). The most appropriate graph depends on what you mean specifically by block growth rate.
