In most cases I paint one object at time. That means I completely finish whichever object I am working on before going on to another object. This does not mean I will not come back to it and make adjustments later when the still life is finished, but I do try to finish one object completely before moving on — and I usually never come back and work on it again.
Sometimes it can be too much to try and focus on more than one object at a time. The other advantage to painting one object at a time is that if your paint is drying or getting sticky, you can let it all dry and then continue later with fresh paint. This is much harder to do if you have objects which  have dried when they were only half-painted.
This is one of many reasons I recommend that you paint in some background color first around whichever object you are painting. That way if your paint does dry on canvas, you can very easily blend into the dry background colors to re-wet those areas and continue with your still life.
B.Alon Brown