I always have a 3 litre bottle of water with a complete bulb of cloves in it, and give this to the birds at least once a week. And when I feel the bulb needs replacing, I pour the water and old cloves into the birds bath
Cheers, I,m sure I,ve read in the past, that providing you rotate the eggs at least once a day, and keep them cool, you can keep them for 2weeks before slipping under another hen, someone may be able to be more precise though