Generally, 6 degrees either side of centre (sometimes up to 8 degrees). Besides factors such as the size of your dish, it really depends on the strength of the transponders, as well as the type (DVB or DVB-S2) of signal. Sometimes, with a really strong transponder you can go out farther. If it is a satellite such as 97W, which has many transponders of varying strength, you usually would want to keep it as a centre LNB, or as close to centre as possible.

Basically, experimentation is the best way determine how far out you can go, and whether you are satisfied with the results.