Here's what my altar did look like- a box I'm working on to sell, a family photograph for my 'heal the past' work, and a few other odds and ends that don't quite have the same meaning for me as when I first set up the altar.
That's better- I moved the box because I'm almost done with it, I slid some items back, I moved the vase to the living room, and I'm now working with dragons. Everything got dusted. The vibe is better now.
How often do I change my altar? As often as I want to. I don't do seasonal decorations anymore. I'm not limited to just one, currently I have three altars and I do some kind of spell work at every single one every day. I have a banishing/protection altar, a family altar, and this one, my personal altar.