If you enter -1 in your bot's autotweet hours field it will autotweet every cycle. How long the cycle is depends on your account, on our Twitter server Gold account cycle about every 10 minutes.
We do not currently have an option for every minute. Twitter has an API usage limit that resets every 10 minutes, so if you access the API for frequently than every 10 minutes you can exceed the usage limit, and every API call will fail.
Twitter also has a realtime API that we are investigating.