China recently announced the decommissioning of "Submarine 303." This was a Type 33 boat (a copy of the Russian Romeo class). Romeo was the successor to the Russian Whiskey class boats, which were, in turn, based on the German Type XXI. The German design first showed up in 1943, and was the first modern submarine, in that it was designed to spend most of its time underwater (with just the snorkel device and periscope above water, to bring in air for the diesel engine and crew). The Type XXI was a 1,600 ton (on the surface) sub, compared to the 1,500 ton Romeos. Russia built over 500 Romeos, while China built over 80. Only about 7-8 of the Type 33s are still in service, used mainly for training. They rarely go to sea.
What was most interesting about this retirement was the official comment that the sub had steamed 38,000 kilometers at sea over its 20 year career. That comes out to less than a week at sea a year. This was not unusual. Chinese subs are not built well, and there have been many breakdowns and accidents at sea. The Chinese have preferred to keep their subs tied up at dock, and have the crew practice there. Not very good training, but it does reduce the risk of losing the boat at sea. And it is good for crew morale.
China has been trying to improve the quality of its subs, and warships in general. They stopped building Type 33s in the 1980s, and began producing 21 boats of an improved design (the Type 35), which they built until the end of the century. These were more reliable boats, and spent somewhat more time at sea than the Type 33s. During the last decade, the Chinese were still having problems with producing reliable diesel-electric boats, and even more problems with nuclear subs. But eventually, the Chinese will solve the quality problems, which is exactly what they planned to do all along.