My soil grows went as much as 6-8 days without water when they are in mid to late veg. You may be able to get away with doing nothing. But, that depends on a bunch of factors. Soil in large pots can probably go the longest without water. Coco in smaller pots probably can't go much more than 3 days or less without water. Lighting, temp, humidity and other factors all make a difference. Google this issue too. I've seem cheap hacks using 2 litre soda bottles and gravity. I just don't remember the details.