I do not recommend a pre soak. If you do like to do it, then no more than 18 hours. All that water pushes out the air bubble seeds have to oxygenate the sprout. I place my seeds directly into damp soil,coco, or root plugs and let them sprout. When they are that small I tell newbies to keep it at a 1/2-1 cup water twice a week. Make sure you are pouring around the plant half way between the pots lip. If you saturate the area directly under it only, the roots will not spread. That is the number one goal early, root system.