I had mashed with about 5 gallons for a 10 gallon batch, while it was diluted & tamed down somewhere was plenty of lactic tart/sourness. I had actually planned to use a little lactic acid to enhance the beer some but felt it didn't need it.
Mine was nice and tart after 24hrs but we varied a bit. I used 10% acid malt and pitch lacto(grains) at 119f let drop to 100 and maintained that temp for 24 hours.