Generally stop at 40 units, though you could bypass it on the first offer. Most of the machines also tell you when it is best to take offer, to left of payout screen if you look for it. Ive gotten the 1000 coin offer on 1st and also last offers. For double top dollar you might want to hold out for a bit more, because you have a chance for 2x and 4x offer, again you can check the advice.
The newer 6 coin machines with 2 games on them seem really bad, but there is also advice on when to take offer if you look at the help menu.
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