[RutZap]

This has really annoyed me recently, I've had interviews with companies that expected me to do a test before even seeing them in person (following a phone interview) and their tasks mentioned I should spend around 8 hours on the test. I think this is definitely unreasonable.

I prefer longer interviews where you're working alongside your potential future colleagues. This way you can also find out if you actually want to work with them or not. Interviews should be a two way affair.

[b0rsuk]

The phone and online coding exercises are called pre-screening. Their goal is not to find out if you're a good programmer, but if you can write code at all. This is to save time and see as many candidates as possible.

[RutZap]

Yes, i get the screening process; but asking a candidate to spend 8 hours on implementing a client library for their API at the beginning of the application process is definitely unreasonable. It's not a coding exercise, but a full blown deliverable.