1st- around 10 weeks, after we heard the heartbeat.
2nd- right away, I think it was less than 3 weeks as I had gotten a very early positive (I was pretty sure I was pregnant about 5 days after conception. We were neither trying nor not trying, so it was definitely a possibility, and I just knew).
3rd- I told his cousin, who is close friends with both of us, before I told him, because I hadn't gotten a positive yet and I didn't want to freak him out. We told close friends and his family around the beginning of the 2nd trimester. I haven't told everybody in my family yet.
All 3 times, I told my work about it at around 15 weeks, when I started to show more visibly.
As an aside, I also worked up until the baby was born both times with no modifications, and I intend to do so this time as well barring any unforeseen circumstances. People are usually really accomodating about letting you sit for a minute or get a drink of water or whatever. My job involves working directly with small children with special needs, on the floor or wherever, and while some things were more uncomfortable as time went on, nothing was really unbearable.