Let say I eat a lot of complex carbs. When will those carbs actually be ready to use for energy? What about fast carbs? and proteins and fats?
Depends on what you eat together more than anything - moderately-fast whey protein is traceable in 20 minutes, hits top blood amino acid levels about 90 minutes in and drop sharply thereafter, casein protein takes longer to peak but has sustained release for up to 8 hours afterwards, simple carbs eaten alone hit your blood stream fast - like, within minutes fast, complex carbs can take up to a few hours to digest.
Fat digests the slowest, at an avg. of 8 hours.
Eat fat with anything and gastric emptying slows down, so you stay full longer - pure carb like "fat-free snackwell cookies" rush through your stomach in about 30-50 minutes and leave you ready for more.
I see that Alan Aragon's guideline is to eat a solid meal 90-120 minutes before your workout to give it time to digest enough that it's actually hit your blood stream when you're working; I'd assume that's a good working average.