How much time you spend doing such work will depend in large part on your position within the organization for which you work. Experience Summer jobs or other related work can be very valuable to future environmental professionals. Even when you're in high school or junior high, it's not unrealistic to think you might able to get some experience in a professional environment. Be aware, though, that you won't be paid a lot of money, or you may even have to volunteer. The trade off can be worth it, though, because you'll see for yourself how it is people working in the environment spend their working hours.
Most environmental organizations have volunteer programs.
But as you look for an internship, volunteer position or summer job, try to find something related to the specialty which you eventually want to pursue. If you want to be a wildlife biologist, for example, you might help out a local wildlife rehabilitation center. If you want to be a marine biologist, contact organizations that work along the beaches, cleaning them or staking out wildlife nest areas. A number of organizations offer summer field classes in field ecology and field geology to show how these sciences are at work outside the classroom. When approaching possible employers, be professional, realistic and courteous.