I hate Bubba, Schwartzenguard and Robot boss typical fights with Spaz, as every of them can be easily killed within a single sidekick. Hard to even call it a boss fight, it's rather pressing two buttons.

Use layer larger than 2x3 for the rain, it looks dull. Rain drops in real world aren't in straight lines. And the tilebugs, uh. Carrotus tileset just sucks in this matter. Even the official level designers didn't seem to be able to fully avoid them. However, those vine thingies on the top of the first screenshot look absolutely avoidable. Also, no eyecandy included.

What EvilMike says is true, however may still be useless when you're unexperienced, in which case you just stop after two days, believing your level has everything already and nothing can be added, so not imagining working on it for the next five days. Also, if it's one week strongly depends on the amount of free time during it. If you have all the weekend free and you work on your level all the time, with breaks only for meals, you can create a level in two days. If you're busy all the time, it can take a month as well. Also it strongly depends on the layout of a level and the type of eyecandy, and other things, I believe.