Serialprint and analogRead time
It is quite complex to work out how "long" it will take to print.  Why?


If there is room in the serial buffer then the text just gets placed in that buffer. Takes almost no time at all to print.
You are printing a variable length string. The longer the string the longer it will take to print if the buffer is full.


However:


Each character consists of 10 symbols (8 data bits plus 1 start and 1 stop bit). 
Characters are transmitted at 250000 symbols per second
Thus each character takes 1/(250000 / 10), or 40 microseconds to transfer through the actual wire.


So printing the number 100, which also has \r\n appended to it (characters 13 and 10) would need 5 characters. If the buffer is full it will take about 5*0.00004s = 200µs. If the buffer isn't full, it will take a fraction of that to just copy the data into the serial buffer.