Local challenge frequency will be up to each Faction's FO. We've been in discussion about this lately, and have decided that there will be some flexibility in how frequent the Local Challenges (faction specific) will be. I think Wobnam is going to update this in the main INFO/FAQ thread, but it's basically going to be that there will be a maximum amount of points per month, and the FO can decide how he wants to split them up between challenges. One big challenge for all the points, 4 smaller ones with fewer points, etc.
If you can't participate in each local challenge, you don't have to, as participation in these only affects your personal score. The challenges that we really would like to see maximum participation in will be the global challenges, where factions will compete against each other. These will only occur every other month, so you should have plenty of time to fit them in.
The first one of these will be coming soon, so stay tuned