All plants are different individuals. even if they all came from the same packet and will have their individual schedules and will not grow exactly the same from one harvest to the next. There are too many factors involved with each and every grow that will influence timeframes.
Going to 18/6 would be much more plant friendly, anything more the 18 hours of daylight is going to stress any and every type of cannabis plant. It may well cause this/these plants to mature faster, the reduction of light often triggers a hormonal signal within the plant to get moving and hurry up finishing. It is also a good trick to use with stubborn and continuously re-flowering Sativa dominant strains in order to speed up their maturation. Even going to 11/13 with photoperiod Sativas is advisable to hasten maturation, with no drop in quality or yield.
So, 18/6 as a max will give you healthier autos, again with no loss of quality or yield. 20 hours of light is just too much for cannabis, and is often recommended by people who don't care about their plants or those who don't understand the basic biology associated with cannabis growth and maturation.
Hope this helps,............
Organoman.