美国博士要读几年才能毕业？
CS Ph.D. in the US， on average， takes about 5-6 years to complete， with some taking longer and some completing in less than 5 years. There is no set duration for a phd program as such； it all depends on your research， how much data you need/can gather etc.
Some programs are more ‘formal’ where there are many required courses that need to be taken (and passed) before progressing to do research work towards the degree. These can take up to 3 years of your time， depending on which department’s requirements your graduate program falls under. After these ‘formal’ requirements have been met（which includes passing all necessary exams）， then you would progress onto doing research that ultimately leads to completing your thesis（final project）. This is what typically takes another 2-3 years. Of course， if you get stuck or are behind schedule， then you could easily take longer than this to finish off your Ph.D.
It's important to note that some departments are quite 'hands-off' where students are given more autonomy to choose their own research topic areas. In departments like this， you will often find that Ph.D. candidates who are interested in exploring new technology and related topics will be able to move rapidly through their degrees － in some cases even finishing within the standard 4 year timeframe.
Regardless of what department you opt to study in， make sure you explore all of your options so that you know exactly what type of Ph.D. experience you want to have！