Hi Milli.
I checked my latest copy of "Growing To Show" and according to that the lines serve as a gauge for the water level. In a 4" and 5" Oyama pot if you fill it to line one or two it will not need water again for 5 to 6 days. If you are using the 6" Oyama and fill it to line one or two that should last for 6 to 7 days. Of course a lot of this edpends on your growing conditions and on the type of Oyama it could last longer.
Hope this helps,
Fred in NJ

Aha! That makes perfect sense, thank you very much! I noticed that filling up those outer pots to the top line makes the soil too wet but larger plants probably need more water. Thank you again!

Regards,
Milli

Some of my plants I fill to a higher line, some of them to a lower line. Also depends on how pot bound the plant is, more roots need more water more often. When I go on vacation, I fill them up to the top line but then I'm not going to water again for 1 1/2 to 2 weeks and then I'll go back to the regular watering. If you use a lower line and your soil is still staying too wet, add more perlite to your soil mix, that should help. My plants really love these pots, but I have too many plants to put them all in them. tish