For most people, Middle Gs (second open fingered lip position) are usually easiest and can be played for absolutely ages, however, top Gs (fifth open fingered lip position) are fairly difficult to tone anyway, so the time is strongly decreaced to about one hour or so. Bottom Gs (bottom 1 and 2 fingered lip position) help get the lips tighter again after not having played for a while (eg. a break in a rehersal).
man two- "oh, about an hour or something"
man one- "cool. mine's about the same too"