I think it depends on two factors:
(1) How much you have to improve- if your math and verbal skills are weak/average and you aspire to get 700+, you will have to spend many hours (300+). Others can, with a couple of simulations under their belt, reach 700 effortlessly. If English is not your first language and/or you haven't read much since high school, a good verbal score requires a change of habits. What sections of the newspaper do you read? Do you ever read magazines that feature analytical ideas? Do you sometimes read written work just to note how it is written? If you have always been weak at math, all is not lost, but it takes a sustained effort to master the basics.
(2) How much time you can spend on preparation per week. If you can spare 10 hours per week, going from 500 to 600 might take two months, and from 600 to 700 might take three or four months. A great source of practice questions for the quant are early high school textbooks and mathematics competitions for 12-16 year olds. Of course, GMATPrep and the
OG should your main focus, but if your math background is weak, it doesn't hurt to venture outside these for practice, if only to develop speed and problem solving skills.
In sum, if you feel that you need to score 700+ (i.e. you want to be admitted to a top MBA program and your profile is less than outstanding) and your overall skills are currently not far from average, then spending one year preparing for the GMAT while you work makes sense. I do think that the skills you would acquire would be useful throughout your career. Just find a way not to lose momentum!