What he said.
A mod helps look after the day to day running of the forums, pouring oil on troubled waters, wielding the "broom of correct places" to move posts to the forum they should be in, getting rid of spam posts offering us the chance to buy discounted Ray Bans whilst earning $25000 a day from home as we contact the solicitor of our recently deceased long lost Uncle who owns half of Africa.
We're quite lucky that we seem to escape a lot of it, but there are still the odd one or two that sneak through.
Commitment wise, I'd rather folks were willing to do it for the foreseeable future rather than a couple of months.
If you log in regularly I'd say once you get used to it, it would add at most 10-15 minutes to your sessions here, having a quick scout about, seeing if things are where they should be and the like.
If you're reading the forums, you're noticing things aren't right, the difference being it would be you that actioned things rather than reporting them.
Three does seem to be the magic number, but a fourth would lighten the commitment for those that do say "aye".
Hope that helps clarify things a little.
Aegis