They will flower regardless of light schedule : 24/0 , 20/4 , 18/6 or even 12/12.
Most common is 18/6 all the way thru. I haven't seen much difference in using 20/4 so i use 18/6. It allows multiple stages to run together ( seedlings , vegging and flowering ) in the same grow space.
EXAMPLE :
Short Ryder strains i use 18/6 UNTIL preflower / flower transition THEN 12/12 for bloom.
THE KEY TO RUNNING AUTOS is providing a long " light on " schedule since it is
" hardwired " to grow with a short VEG STAGE ( from seedling to preflower ) in as little as 4-5 weeks from germ. Usually 30 days or so into growth , sexing will show.
And of course .... A good light setup . MORE LIGHT EQUALS MORE BUD
So i say use AT LEAST 18/6 for your autos.