Petitioning will take you perhaps two half-days, depending on how fast you can track down approximately fifty of your neighbors who are registered Democrats.
Once you get on the county committee, your role as a member is as elaborate as you want to be. At minimum, it will require attendance at (very) occasional meetings, perhaps as seldom as once per year. But some people like to use it as an opportunity to organize their neighbors around issues that matter to the community. In that case, the sky's the limit!