I started my first long-awaited Rit project last night (with my HHKBP2), and so far it's been terrific. There's more to come, and I'll provide pix when I'm finished.
Some notes, if it helps:
You do not need to use the whole pack of dye powder. Certainly, the more you use, the faster it will take, and there appears to be a linear (or at least linear-ish -- I didn't collect data) relationship between the amount of die you have in the pan vs. the amount of time required to achieve the deepness of color that you desire. There also might be an issue re: "depth of dye penetration", which seems proportional to the amount of time that the keys are in the dyebath, but that's speculative.
I don't believe that temperature is too much a factor in this equation. It helps to get it warm/hot, but other than that, just be cautious and keep it from boiling. However, another variable is likely "how much plastic are you dying in a given run", but I was only doing two keys each time, so I don't have any data for that. I'll be doing some in bulk later, hopefully tonight.
The instructions state that one pack is suitable for three gallons of water (which is suitable for a large volume of fabric, I forget the actual amount; I do believe that fabric takes dye better than plastic, however). I used just under a quart of water, and while that would imply that I should use about 1/12th of the packet, I actually used about a third. To help fix the colors, I also added appx 1-2 tablespoons of salt, as per the instructions.
I kept the water just under boiling (with the burner set at about medium; YMMV), and it took 20-30 minutes to get my preferred deepness of color. So far, I turned my function key "Golden Yellow", and my escape key "Scarlet". The yellow tends toward rusty red if it goes too long, while scarlet tends toward a maroonish black.
My wife (a chemist-turned-biostatistician, and a Rit tie-dyer in her undergrad days) advised that Rit dyebaths shouldn't be done in cookware that you intend to eat out of again. (The Rit instructions, however, merely state that everything should be washed very throughly afterwards with lots of bleach, so take your pick.) Personally -- because we share the same cookware -- I used a pan that I picked up specifically at a thrift store for US$3, and for agitating the water, I used a pair of disposable chopsticks.
How did I know when to stop?: I used the chopsticks to frequently dig out a key to appraise its color, and stopped when it seemed right.
Many thanks to iMav for starting this fun trend. So far I've been having a ball.