The flash off = The MEK/Acetone will evaporate from the melted plastic. Time varies with the initial mixture, temperature, and other environmental variables.

Bottom line, one day, 3 days, a week, you'll know when it's ready to sand when the plastic returns to a hardened state, if it's not fully cured it can gum up while being sanded.

When you break down ABS this way it does change it's chemical composition somewhat, it will lose some of it's "stiff" properties and stay just a tad to the flexible side (a good thing when fixing cracked/stressed areas) so don't expect it to return to a rock hard state, just close to it.

Also keep in mind that the glueing or welding portion of a plastic repair job is the easy part, the real, skill, time, and effort falls into the sculpting, sanding, priming, painting, etc. Just like body work in miniature. Practice, practice, practice..