If you can keep your chocolate at its working temperature it will stay in temper for many hours. However over time it will become thicker as the beta crystals continue to multiply. Once your chocolate is in temper you can reheat it to keep it liquid enough for molding. Just make sure you don't let it get above the working temperature.
White chocolate contains a lot of milk fat along with cocoa butter. Milk fat crystallizes differently than cocoa butter and makes tempering white and milk chocolate more challenging than tempering dark chocolate. White chocolate is more susceptible to burning than dark chocolate due to the milk solids in it. The temperatures involved in tempering white and milk chocolate are in general 2 to 4 degrees C lower than those for dark chocolate.
When molding with white chocolate, make sure you test the temper of your chocolate and don't start molding until you are sure the chocolate is in temper. While you generally want a nice thin shell when you are molding with dark chocolate, with white chocolate a thicker shell will give a better result, especially if you are using thinner, more liquid fillings. Cool in fridge just long enough to set, don't leave in fridge for extended periods as this seems to encourage condensation and results in a dull finish.
We have a small marble slab for tempering. I was wondering if tempering on a stainless steel table would be possible.
A stainless table doesn't have quite the same thermal mass as marble. So cooling is not going to be as effective as marble or granite.