flying to Kuala Lumpur with Airchina from Stockholm and we have a stopover in Beijing for 7,5 hours

We are thinking of going to the city centre for a couple of hours. Does this mean that we need to apply for 24-hour or 72-hour transit visa? And if so, how fast is the process for European citizens (most specifically, from Greece and Sweden). Will it take1,5 hrs as I ve read somewhere or since our stay there is less than 24hours we don't need a visa?
Thanks in advance.