Some people have the practice of meditating without setting a time limit.
There are, however, certain problems with this. The problems arise because, if we meditate until we are too agitated or too dull to continue, then we are only reinforcing agitation and dullness.
It is better to NOT have a harmful or negative mind-state be the determining factor for ending a meditation session. Our minds are tricky and will use this opportunity to generate harmful mind-states and reinforce them.
That’s why it is better to set a time limit. It can even be a limit such as meditating until breakfast or lunchtime or meditating until a guest arrives, anything that is not governed by our state of mind.
Always remember that it is better to have many small sessions than to have a long session with a long gap between. A lot of progress can be made by just 5 or 10 minutes of meditation each day and it helps to always enjoy the practice so that we are tempted to return to it.