When I was on consistent (same city, regular, repeated flights) projects I would book anywhere from 1 to 3 months out. Our company was generous in that they'd reimburse flights at booking time as well as pay change fees. It was a smart move for them: I'd sometimes book 6-8 tickets at the same time during a good fare sale. I was working in DC for 2 years: there were be days where I'd book a whole stack of $125 all-in R/T's to/from Kansas City and get reimbursed for them all on one expense check. If I ended up not flying home for one of them, or having to change something at the last minute, it still netted far lower overall costs in the long run.

Of course, they were all bought as Thursday outgoing - Monday return trips. I guess by now everybody knows to do that (with the first day on the job being with a one-way ticket out there).