I usually do the same as mcoov, watch other brickfilms or listen to music.

I don't to how much animation, I do it by time. I'd advise anywhere from 1-2 hours, depending on how much you want done, how fast you can animate, and how soon you want the film out.

Add little details to your sets to make them really stand out from just being a plain, boring wall or something.

As mcoov said, it's entirely up to you, but I'd probably make something in the 1-2 minute zone.

Watch some really awesome brickfilms right before you start animating. Also, I find that if I storyboard a film, I stick with it a lot better than I usually do. But when it really comes down to it, I get inspired by saying "Alright, I haven't made a brickfilm in a while, I'm going to drag myself from whatever I'm doing and make one." Sometimes you really don't feel like brickfilming, but once you initially push yourself off of the computer chair or wherever you are, you get a lot more in the mood for it.

It helps me if I can find one person to work with, most of my films were made with a friend, we both animated, he did music, I did digital effects. Even if they are just there to help you build sets you can bounce ideas off them, sometimes chatting with other people helps.

I would try to write a script, and if possible keep those early scripts to one page. Keeping films short will make it easer to finish. I started one very large project and never finished it, I still have about 52 seconds of great looking animation from it, but I don't think it will ever get finished.

We get inspired by curiosity, we usually watch Picturesque, Unrenewable, Grace, Frankenstein, Driven or The River, and we ask the question, "How do they do that?" And then we start animating.