What is maximum length of time this will gather information?

As best as I can tell, it will collect information until it is unplugged or loses power. The time can be set if you fiddle with the function keys. However, once it is unplugged, or suffers thru a power outage, it resets back to all defaults. This means the time resets to 00:00 hours and it restarts collecting a running count of kilowatt-hours. It will collect information as long as it has power. I bought this thing because it has a backlit display. Unfortunately for me, the display turns off after 30 seconds. I can press one of the Up or Down buttons to reactivate the back light, but it still only stays lit for 30 seconds.

Is it possible to get a manual?

There was a piece of paper in the packaging that is indicated to be the "manual." I'm not aware of a separate manual that can be downloaded or purchased.

Anyone know what the warranty on this might be?

I believe if it doesn't work, they'll replace it (DOA)... Be warned...the brighter LCD (backlighting) comes on when you push any of the buttons and only stays on for about 20-30 seconds then goes out; you cant set it to stay on all the time. When it is out, you have to be about a foot away from the display and directly in front of it to read it. I used mine for about a week. It is now unplugged sitting on my workbench collecting dust.