How often is a block mined on the bitcoin testnet? I've been watching testnet for a few minutes and I haven't seen a block come in yet. I'm testing some code that works with incoming validated blocks, and I'd like to test it out.
Blocks on testnet3 are intermittent; sometimes there's hundreds of blocks a second, massive reorganisations that strike out tens of thousands of blocks, or no blocks for many hours. This is largely due its zero value and the difficulty adjustment rules that don't deal well with changes in hash rate. Generally no software is tested here other than for public demonstrations as a result. 

For more reliable testing the regtest mode exists in Bitcoin Core which allows you to instantly create blocks on demand in your own isolated testnet with as many peers as desirable.