What would you say is a reasonable amount of time a tech test should take?

A take home task - no more than a few hours (an evening or twos work max imho.) For an in-person interview task, half an hour. Many would say longer, but I'd say any well designed interview task should be able to get enough info out in that timeframe that, while watching, you know all you need to.

Of course, both of the above are poor substitutes for seeing if someone can actually program well and fit into your team - but that's par for the course with interviews anyway. It'll always be a best guess.

I've been given one that's 4 hours long and it has to be submitted in that time. Just think it's a bit too much :/