I love the idea of rainwater. Around here care does have to be taken about standing water breeding mosquitoes. So a big open bin wouldn't work without treating the water.

How far past 4 days can you realistically go.

This is brand new, I have no idea. We're finding out.

I was gonna buy one of those sprinkler auto watering attachments for my 2 week vacay

A concern here is that the lawn people might accidentally cut any lines left out above ground, but if you don't have that issue, I'm pretty sure you can hook drip irrigation to an outside faucet and you wouldn't have to worry about running out of water.