Way too little watering - what should I do?

If these are the plants from your diary that you are asking about, I agree, 3-5 litres every few days would be about right. Deep watering to run off every few days is much better than lots of little waterings every 2-3 days. You need run off to prevent the building up of nutrient salts to toxic levels in your soil, that will then harm/burn your plants. The best moisture meter is your finger, stick it in the soil and once the top 2-3 cm are dry, this is when to water deeply again. This may be anywhere between 2 and 7 days, depending on temps, your plants requirements and the water holding capacity of your soil mix. Hope this helps, Organman.