At previous employers (where I was able to expense my tix immediately after booking), I would book 4 weeks in advance in order to minimize airfare costs. My current employer does not allow me to be reimbursed for airfare until I have actually flown the flights. I now book <2 weeks out (just booked tix for week of 2/12-2/17). This allows me to not have to float anything to Diners Club, as my expense items paid w/ DC are paid directly to them after I submit reports w/ receipts.

How far in advance do you book your tix while on longterm projects?