Points to be aware of:
1. Promotion boards only give credit for activities they know about. So, if you're doing meals on wheels etc, make sure that your reporting chain are aware of it throughout the year and include it in your OJAR. The more of your time you spend on an activity, the more you should expect to see it highlighted in the report.
2. If all you can demonstrate is 1 evening a week, be certain that there are plenty of others on your promotion board doing a helluva lot more and so getting their extra-curricula work highlighted more strongly.
3. Secondary duties aren't what get you promoted, it's just the factor that puts the top of the board in a particular running order, because you have to use something to differentiate between 2 very good workers who are neck and neck professionally (or, to be accurate, appear to be neck and neck based on their reports). I have sat on boards where we have only had 1 slot to fill and something had to decide the order between 4 excellent candidates, all of whom appeared to be god's gift at work. (As an aside: 1 got bumped down because of a single 'not glowing' comment from his 3rd RO; 1 was dropped because although they had spec recs, and had done so for 3 years since promotion, we got the gut feeling from the 1st RO's narrative, that the subject was living on past glory and only got a spec rec because they got one the previous year; and we could have tossed a coin between the final 2, deciding on the 'winner' because their 3rd RO had used a phrase along the lines of 'best I have seen in 34 years of service' whereas the other's said 'the best I have seen.' Harsh, but the board has to make a decision somehow. Incidentally, a bloke with shed loads of secondary duties didn't get placed in the top 5 as the report didn't extol his virtues at work sufficiently.)
4. Boards don't care what you do out of work as long as you are showing some dedication, a willingness to take additional responsibility and don't just sit around watching the telly all the time. Almost any activity can show those qualities.
Added for Tigermate:
The time varies depending on the board and the place you come on the board. The first people to be read are done slowly as the board 'finds its feet' and will be looked at again at the end to make sure that the same yardstick has been applied throughout. Smaller boards get longer readings per person than larger ones. On average, each candidate probably gets 5-10 minutes (remember that not everyone of all grades get read).