Check the humidity at least once a day (may not be enough on hot summer days). If dry, water until it starts dripping out the bottom. Only water when the Bonsai is dry, avoiding dry periods longer than half a day. Especially in summer the roots may be severly damaged through lack of regular watering.
By watching your bonsai closely you will find out about its thirst and its own watering schedule - and yes, you could compare bonsai somehow with a pet.