Distance undersea that a submarine can travel today?

Today's Nuclear Powered Submarines' underwater cruising range is limited only by her crew. Without the need to resupply food, modern nukes could remain submerged indefinitely.

During my own time as a Submariner, another boat in our Squadron, USS Seahorse (SSN-669) completed a full circumnavigation of the globe, posting 180 days submerged. For any submarine crew, that's a long time - even SpecOps boats like Parche were only gone and submerged for 4 months. My own personal submerged record is 72 days.

Eventually in the future, the food barrier will be broken, but the psychological effects of working in confined spaces for months will not. After a couple of months, the air gets a bit stale, even if it is constantly replenished and purified. Due to the nature of submarine operations, snorkel operations aren't always a option to ventilate the boat with fresh air. Over time, stress and pressure begins to take its toll on even the most professional submariners. It is just a fact of human nature, but also a testament to those who can do it.