Apparently you have figured out that three days is way too long for any thing let alone bondo. Hey we all have to start somewhere. Try to mix close to the manufacturers recommendations (it is an eyeball thing). Try to get out as much of the dent as possible. Work the bondo high to the center of the dent and feather to the edges when applying. If you are new to body work, when sanding, don't judge by the eye, feel with the hand. (Note that bare hands on sand paper will usually end in poor result).If you think your close, spray with gray primer and (mist or cloud over) with a black or a darker primer, sand with a sanding block or foam block. Your high and low spots will appear quite fast. If it is on a flat surface, use a sanding board (pneumatic) or a flat short board.
If you mix bondo with too much hardener it can crack and not to mention waste your money and time because it also gums and balls up very quick while you are trying to spread it.
If you have to fill a deep dent, IE: You can't knock it out or have no access to a dent puller (the kind you can mig a "nail" to the metal) is by far better than, the make a hole by attachment method. Conside dynaglass (green fiberglass filler). Do it in layers and allow for good cure times. The worst thing on any finish is bulging or popping of bondo work.
