ACI meets nationally twice a year, and with few exceptions, each committee meets for 2-4 hours on one day only. The 318 and 301 committees are very closely held, and getting on these usually requires prior participation on other committees. There are 4-5 days of meetings during each convention. Outside of conventions (which you need to attend at least once a year), committee work varies and you can quickly get dragged into frequent conference calls and writing proposed language. BUT, many committees work very slowly and do very little outside of conventions.
ASTM meets twice a year, and involvement can be as much or as little as you like, but if you join and participate, please be sure you vote on ballots, to assure that the process works as needed.
ASCE committees vary widely as to the amount of commitment required, but I am only starting to get involved with ASCE committees, so I can't speak intelligently about them.
For all of these groups, bear in mind a few things:
1) people paid to participate tend to have more time, which means industry representatives (like me) and academics tend to dominate the conversation. However the committees require a certain balance, depending upon their ANSI accreditation, so your voice is important and needs to be heard. I am in the somewhat unique position of having been a designer recently, so I don't only represent the industry, but also view this work from the position of a user.
2) There is often longstanding politics and history that should be respected, since proposed changes sometimes just reverse the course - sometimes for the better, sometimes because the underlying reasons are not apparent but make sense in the greater context. If you want to reverse the course, be prepared to politic and work behind the scenes to build consensus.
3) Get involved and stay involved, so you become the constant voice of reason. Don't expect to make sweeping changes quickly. It is frequently a game of survivor, and the people who consistently participate and work hard tend to get their items adopted (if the proposal stands on its merits.)
In the end, the people who are the constant face of the committee act as the elders and the voice of reason when the guys like me proposed crazy, big changes (like simplification of the development length equations.) Good committees work well with a mix of new and old members, and fresh voices bring good perspective and valuable input to the committee's work. It is very easy to get over-committed, so go cautiously.