My advice would be to ignore how dry the soil is at the top - it will always dry out first there.. With that size of pot I would suggest every 4-5 days, but LIFT THE POT when you've given them a good soaking, try to remember how heavy it is, then lift it after 3/4 days and see what if feels like - if its light, water. Simple as that.

I wouldn't worry about lower leaf yellowing, as firetheft says, its normal..

MJD