There are no hard and fast rules about the time period as the same varies according to demand. In general it can be said that the tickets should be bought at the earliest once a definite travel program is finalized. Also besides the website of travel agencies, the websites of the airlines should also be checked for the prices of the tickets. Generally non-refundable tickets are sold at cheaper rates. In case it is not possible to conduct research on one's own, then the services of the travel agents should be availed.