A long time, shortened. English speakers are quite lazy, we shorten a lot of things. It doesn't take long to clean my apartment. It doesn't take (a) long (time) to clean my apartment. I don't take (a) long (time) in the shower. Don't be (a) long (time) walking the dog.

It means that you should not spend a lot of time outside. It could also mean that you should finish something quickly. Basically, it means be fast or to do something quickly.