What prey for how long

Know this info is probably somewhere in here already but is there a general guide to what size mouse/rat to feed for how long before moving up? Thx

Most go by the weight of the snake feeding 10-15% of the snakes weight in the rodents weight. Other than that you can just eyeball it and feed prey that is near the same thickness of the snakes thickest portion of its body. Each snake can be different and depending how often you feed your snake will make the rate of growth change.

Yeah I know same thickness as thickest part of snake but I mean like that's it just buy rats based on that doesn't matter what they are?

Rats are rats. As long as they are from a reputable place and the proper size all is well. If you buy in bulk and the snake outgrows what you have stocked up you can feed more frequently or double up if it equals the correct size meal.