The average adult attention span isn’t very long. Depending on where you look you will get varying answers as to how long people can focus for, but generally, the average adult attention span is less than half an hour.
People often try to force themselves to focus for long periods of time, but that is just asking for failure. They quickly get distracted and lose focus.
The same can be said for long-term focus - also known as life goals and ambitions. People make New Year’s resolutions every year. For the first few days, their focus is strong. Eventually, they inevitably lose focus and stop pursuing their resolutions.
One solution to increase your focus is to instead work in short bursts of concentrated, focused intervals, and then taking a short break where you disconnect from the work. This is a lot more sustainable than one long period of sustained focus.
The same solution can be applied to long-term goals. Instead of having one big goal or ambition that you are supposed to work towards in a continuous manner, break your goals into smaller chunks, and focus only on those goals. When you achieve a goal, take a bit of time to celebrate and appreciate your work. Then move on to the next goal.
For both short and long term focus, the key is to break things down into smaller chunks, and focus on completing each chunk at a time, taking breaks and pacing yourself along the way. In this way, you train yourself to focus, and will find that you can sustainably focus for longer.