IMO: Think of gallons like months in anything except DWC. I can tell you for sure that 2 gallon DWC bucket root balls will be choking out air stones and need heavy manicuring if you so choose.
Only time I would 12/12 off the top is in a production style grow with output every 2 weeks to make up for lack of quantity. I would do 3.5 to 7 gal tall buckets if you have the vertical for the 7.
I have done 2 gallon DWC and the roots consumed the entire interior, air stone and all had to cut it out and only took .75 gal to fill or something. Water temps will be warm unless you pipe everything. It becomes kinda absurd how big it gets in there.
I vegged them 3 weeks under LED's so they were short and bushy but root zone was explosive, then put them under HPS/LED for flower.
Looking back that is kinda random sorry.