To hit minimum fitness guidelines for aerobic exercise and resistance training, do something every day, every even if it's a moderate intensity brisk walk. Those walks can be enough to meet minimum aerobic workout guidelines or can be a good recovery day if you are going harder other days.
On days where you don't have a lot of time, know that it's still possible to make significant progress with a short, intense workout.
But there are benefits to working out for longer periods of time too, which can help you burn more calories and might be beneficial for brain health.
If you really want to improve at something, do it at least three days a week.
Finally, turn fitness into a lifelong habit, as the longer you do it, the more benefits you will see.