Sampling at 15 / sec (worst case in your specification) requires a budget of 66mS / sample.
Allow some time for pre-processing in your code of the same, lets say 5mS
Allow some time to get the sample out on the serial port at 9600baud say 10mS per sample (1/9600 * 10 bits x 10 characters e.g. SEROUT ("V = 3.10 v")
Allow some time to go back around the loop to start the process again and do some simple tests, like are all samples complete, has the routine finished, lets say 5mS for that
Leaving a total budget of 66 - 5 - 10 - 5 = 45 mS for your ADC to do the conversion. Someone on this forum, I can't find it just now, has determined the conversion time for an 8 or 10 bit conversion, which in this example needs be as fast, if not faster than 45mS.
All this leads to a formula that you can re-arrange to give you baud rate and determine the maximum speed, or conversely fine tune all of the numbers to optimise your result for sampling speed as required. If you produced a simple draft sampling programme you can use the timing table values for each command in this forum to determine your maxmium speed. It looks like ADC conversion time is the determinant.