Don, I'll try to give you some useful answers...
...what is the minimum time needed to get a positive switch?
Time isn't a problem. The crucial point is the size of the capacitor.
At first the capacitor is charged - compare this to a water tank which is filled up. When you press the button, a circuit through the switch solenoid is closed and the capacitor is discharged, thereby actuating the turnout. (You are tipping the tank, so to speak, so all its content is spilled in one big splash!)
Now the current (in Amperes) depends on the voltage of the capacitor (as Shamus said, 24 V is fine) and the electrical resistance of the solenoids. In a setup like the one I built up, the discharge of the capacitor gives a burst of current which lasts for maximum some tenths of a second. Since you are depressing the button only for an instant, there is no danger of burning out the switch machine(s). And as soon as you release the button, the capacitor is recharged again (which is done in a few seconds).
...what is the maximum time before you start smelling smoke?
Capacitor size is important, too. If you take a giant capacitor, there is more electrical charge - and it could be too much, since the discharge current flows longer and this could build up some heat in the solenoids. So don't use an oversized capacitor, and there should be no smoke problems! (Compare again: You could clean your porch by gushing a bucket of water over the planks. You wouldn't gush the same porch with a railroad water tower, would you?
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I found that the size shown above (4700 Microfarad) is more than ample for the actuation of up to five turnouts together. For one or two turnouts at a time 2200 Microfarad should suffice.
In the charging circuit you could include an additional resistor to keep loading current down. This gives some additional security, but so far I had no problems without.
Important: Use a push button, never a switch! Otherwise you'll have an ongoing current through the solenoids - and in this case they could burn out rapidly. In this case you would smell some smoke!
And BTW: Take care when wiring an electrolytic capacitor: When you reverse the polarity, it explodes. It's not so much the danger of flying debris - but the thing STINKS, oh Boy! (I know
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Ron
PS: Wow, this answer became somewhat king-sized, but I hope it helped a bit...