In general, I practice hours and hours a day. Getting good at Jazz is all about getting good at practice. Practice literally becomes a meditative act, and something that transcends all else in life. You have to be the type of person who gets separation anxiety just having to put down your instrument for even a few hours. Practice is so important that Jazz great Sonny Rollins actually quit playing for some time at the peak of his career just to have time to go out and woodshed under the Williamsburg Bridge.
This said, there are a few important things to understand: First, consistency is most important. Believe it or not, 5-10 minutes a night of consistent practice will actually work more than going most the week without practicing, then cramming all night or day before the lesson. This is in part because it takes time for things to sink in. Cramming does just about nothing for muscle memory and development, and since in music over learning is learning, just sitting down at your instrument for a couple hours one day a week isn't really going to do much to turn you into a confident performer.
So, what it the best way to practice? First, you really want to have a clear picture of what you're working on. Consider your teacher a type of performance coach who teaches you how to practice. Do not pay to take a lesson without leaving with a clear idea just WHAT exactly you have to practice and how you're going to practice it. Sure, there are lot's of neat and cool things your teacher can show you, but in the long term, its all about preparing you for the week of study ahead.
Related to this, be wary of teachers who lack focus and provide very complicated homework assignments as if the lesson has a million different points. Remember, even a simple set of just a few repetitive exercises may require hours and hours of practice to get to the point that the marks are reached. Good teachers don't bury the main points of their lessons: they enforce and reinforce them. Furthermore, good teachers set goals their students can accomplish then build on those successes. They don't overwhelm their students with unattainable goals from the get go.
Practice and Binary Achievement You might say, "Wait, how am I going to get ahead if the teacher only gives me baby work to do between lessons?" First, we all know the story of the tortoise and the hare. Second, music isn't just about filling up a bag. The learning is more binary, so you learn something simple, then you build on what you've just learned, then you build on that, and suddenly what looks like just 3 simple tasks to do, turn into the embodiment of days, weeks, months or years of study, the only catch is that once the material is internalized, it doesn't feeldifficult or overwhelming.
As for practice, I always end my lessons by reviewing what my students are going to practice for the next lesson, and also how much time they'll be putting in a night. After setting the plan, I ask, "O.K. suppose you're super busy and don't have time for more than 5 minutes of practice a night. What are you going to do?" We then work out a basic regiment to adhere to on a daily basis no matter what. It's just 5 minutes after all.
Now, you may wonder, "Is 5 minutes really enough to get good?" -To sound like John Coltrane? No. But think of the hare and the tortoise analogy. Our concern here is constantly moving forward. The reality is that most adult students are going to have difficulty practicing at times. Accepting this, my job as the teacher is to make sure that they're going to be able to strategize practice time to the point that some kind of progress is possible, even on the busiest of weeks. So long as practice, even "micro-practice" is possible, the student will make progress, and again, since musical progress is binary, results will be compounded.
In the end, my final advice is this: Even if you don't have a very limited amount of time, you can still make progress by developing a practice regime that works for you. On the other hand, don't get carried away with this newfound liberty: The fact is plain and simple. You still need to put in the work.